diff --git a/LinqStudy.Test/ExpressionTree/表达式树.md b/LinqStudy.Test/ExpressionTree/表达式树.md
new file mode 100644
index 0000000..bad650c
--- /dev/null
+++ b/LinqStudy.Test/ExpressionTree/表达式树.md
@@ -0,0 +1 @@
+# 表达式树
\ No newline at end of file
diff --git a/LinqStudy.Test/LinqStudy.Test.csproj b/LinqStudy.Test/LinqStudy.Test.csproj
index 86d69ce..b4c1c42 100644
--- a/LinqStudy.Test/LinqStudy.Test.csproj
+++ b/LinqStudy.Test/LinqStudy.Test.csproj
@@ -18,10 +18,6 @@
     </PackageReference>
   </ItemGroup>
 
-  <ItemGroup>
-    <Folder Include="ExpressionTree\" />
-  </ItemGroup>
-
   <ItemGroup>
     <ProjectReference Include="..\LinqStudy\LinqStudy.csproj" />
   </ItemGroup>
diff --git a/LinqStudy.Test/LinqToObject/ElementTest.cs b/LinqStudy.Test/LinqToObject/ElementTest.cs
index 3918a6a..0e1e697 100644
--- a/LinqStudy.Test/LinqToObject/ElementTest.cs
+++ b/LinqStudy.Test/LinqToObject/ElementTest.cs
@@ -451,5 +451,114 @@ namespace LinqStudy.Test.LinqToObject
             Assert.Throws<InvalidOperationException>(singleOrDefalutAction);
         }
         #endregion
+
+        #region ElementAt
+
+        /// <summary>
+        /// ElementAt：返回集合中给定索引(索引从0开始)处的元素，延迟执行。
+        /// </summary>
+        [Fact]
+        public void ElementAt_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+                new Person(){ Id=3, Name="wangwu", Age=44},
+            };
+
+            var queryItem = peoples.ElementAt(0);
+            var queryItemName = queryItem.Name;
+
+            Assert.Equal("wanggaofeng", queryItemName);
+        }
+
+        /// <summary>
+        /// ArgumentNullException异常：数据源为Null时，引发。
+        /// </summary>
+        [Fact]
+        public void ElementAt_ArgumentNullException_Test()
+        {
+            List<Person> peoples = null;
+
+            Action queryAction = () => peoples.ElementAt(0);
+
+            Assert.Throws<ArgumentNullException>(queryAction);
+        }
+
+        /// <summary>
+        /// ArgumentOutOfRangeException异常：指定索引超出序列范围时，引发。
+        /// </summary>
+        [Fact]
+        public void ElementAt_ArgumentOutOfRangeException_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+                new Person(){ Id=3, Name="wangwu", Age=44},
+            };
+
+            Action queryAction = () => peoples.ElementAt(100);
+
+            Assert.Throws<ArgumentOutOfRangeException>(queryAction);
+        }
+        #endregion
+
+        #region ElementAtOrDefault
+        /// <summary>
+        /// ElementAtOrDefault：获取指定索引处的元素或如果超出索引范围时返回默认项；延迟执行。
+        /// </summary>
+        [Fact]
+        public void ElementAtOrDefault_ElementAt()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33}
+            };
+
+            var queryItem = peoples.ElementAtOrDefault(0);
+            var queryItemName = queryItem.Name;
+
+            Assert.Equal("wanggaofeng", queryItemName);
+        }
+
+        /// <summary>
+        /// 超出索引范围，返回默认项。
+        /// 注意：引用和可空类型，返回null
+        /// </summary>
+        [Fact]
+        public void ElementAtOrDefault_Default()
+        {
+            List<Person> peoples = new List<Person>() { };
+
+            var queryItem = peoples.ElementAtOrDefault(2);
+
+            Assert.Null(queryItem);
+        }
+
+        [Fact]
+        public void ElementAtOrDefault_ArgumentNullException()
+        {
+            List<Person> peoples = null;
+
+            Action queryAction = () => peoples.ElementAtOrDefault(0);
+
+            Assert.Throws<ArgumentNullException>(queryAction);
+        }
+
+        /// <summary>
+        /// 查询不到数据，不异常
+        /// </summary>
+        [Fact]
+        public void ElementAtOrDefault_QueryEmpty_NoArgumentNullException()
+        {
+            List<Person> peoples = new List<Person>();
+
+            var queryItme = peoples.ElementAtOrDefault(5);
+
+            Assert.Null(queryItme);
+        }
+        #endregion
     }
 }
diff --git a/LinqStudy.Test/LinqToObject/QuantifierTest.cs b/LinqStudy.Test/LinqToObject/QuantifierTest.cs
new file mode 100644
index 0000000..0d13c7b
--- /dev/null
+++ b/LinqStudy.Test/LinqToObject/QuantifierTest.cs
@@ -0,0 +1,307 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Linq;
+
+using Xunit;
+using FluentAssertions;
+
+
+namespace LinqStudy.Test.LinqToObject
+{
+    /// <summary>
+    ///  量词操作符
+    /// </summary>
+    public class QuantifierTest
+    {
+        #region Any
+
+        /// <summary>
+        /// Andy：序列中是否存在任一满足条件的项；为加快判断速度，立即执行
+        /// </summary>
+        [Fact]
+        public void Any_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id = 1, Name = "王小明",Age = 10 },
+                new Person(){ Id = 2, Name = "西站热",Age = 66 },
+                new Person(){ Id = 3, Name = "西门吹雪",Age = 45 },
+            };
+
+            var anyQuery = peoples.Any(q => q.Name.StartsWith("西"));
+
+            Assert.True(anyQuery);
+        }
+
+        /// <summary>
+        /// 立即执行
+        /// </summary>
+        [Fact]
+        public void Any_Immediately_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id = 1, Name = "王小明",Age = 10 },
+                new Person(){ Id = 2, Name = "东方不败",Age = 66 },
+                new Person(){ Id = 3, Name = "西门吹雪",Age = 45 },
+            };
+
+            //查询
+            var anyQuery = peoples.Any(q => q.Id >= 4);
+
+            //查询后添加满足条件的项
+            peoples.Add(new Person { Id = 5, Name = "小东", Age = 44 });
+
+            //结果
+            var delayResult = anyQuery;
+            //断言
+            Assert.False(delayResult);
+        }
+
+        /// <summary>
+        /// 重载方法：查询表达式
+        /// </summary>
+        [Fact]
+        public void Any_Overload_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id = 1, Name = "王小明",Age = 10 },
+                new Person(){ Id = 2, Name = "东方不败",Age = 66 },
+                new Person(){ Id = 3, Name = "西门吹雪",Age = 45 },
+            };
+
+            var anyQuery = peoples.Any(q => q.Id >= 2);
+
+            Assert.True(anyQuery);
+        }
+
+        /// <summary>
+        ///  ArgumentNullException异常
+        /// </summary>
+        [Fact]
+        public void Any_ArgumentNullException_Test()
+        {
+            List<Person> peoples =null;
+
+            Action anyQueryAction =() => peoples.Any(q => q.Id >= 2);
+
+            Assert.Throws<ArgumentNullException>(anyQueryAction);
+        }
+        #endregion
+
+        #region All
+
+        /// <summary>
+        /// All：序列中是否所有项都满足条件；
+        /// 立即执行
+        /// </summary>
+        [Fact]
+        public void All_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id = 1, Name = "王小明",Age = 10 },
+                new Person(){ Id = 2, Name = "王小庆",Age = 66 },
+                new Person(){ Id = 3, Name = "西门之王",Age = 45 },
+            };
+
+            var anyQuery = peoples.All(q => q.Name.Contains("王"));
+
+            Assert.True(anyQuery);
+        }
+
+        /// <summary>
+        /// 立即执行
+        /// </summary>
+        [Fact]
+        public void All_Immediately_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id = 1, Name = "王小明",Age = 10 },
+                new Person(){ Id = 2, Name = "王小庆",Age = 66 },
+                new Person(){ Id = 3, Name = "西门",Age = 45 },
+            };
+
+            //查询
+            var anyQuery = peoples.All(q => q.Id >= 4);
+
+            //查询后设置满足条件的项
+            peoples[2].Name += "之王";
+
+            //结果
+            var delayResult = anyQuery;
+
+            //断言
+            Assert.False(delayResult);
+        }
+
+        /// <summary>
+        /// 重载方法：查询表达式
+        /// </summary>
+        [Fact]
+        public void All_Overload_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id = 1, Name = "王小明",Age = 10 },
+                new Person(){ Id = 2, Name = "东方不败",Age = 66 },
+                new Person(){ Id = 3, Name = "西门吹雪",Age = 45 },
+            };
+
+            var anyQuery = peoples.All(q => q.Name.Length >= 2);
+
+            Assert.True(anyQuery);
+        }
+
+        /// <summary>
+        ///  ArgumentNullException异常
+        /// </summary>
+        [Fact]
+        public void All_ArgumentNullException_Test()
+        {
+            List<Person> peoples = null;
+
+            Action anyQueryAction = () => peoples.All(q => q.Id >= 2);
+
+            Assert.Throws<ArgumentNullException>(anyQueryAction);
+        }
+        #endregion
+
+        #region Contains
+
+        /// <summary>
+        /// Contains：序列中是否包含指定的项；
+        /// 立即执行
+        /// </summary>
+        [Fact]
+        public void Contains_Test()
+        {
+            List<int> arr = new List<int>() {1,2,3,4,5 };
+
+            var anyQuery = arr.Contains(2);
+
+            Assert.True(anyQuery);
+        }
+
+        /// <summary>
+        /// 不包含时，返回false
+        /// </summary>
+        [Fact]
+        public void Contains_No_Test()
+        {
+            List<int> arr = new List<int>() { 1, 2, 3, 4, 5 };
+
+            var anyQuery = arr.Contains(100);
+
+            Assert.False(anyQuery);
+        }
+
+        /// <summary>
+        /// 引用类型的包含，指引用的对象是同一个对象。
+        /// 属性值全部相等，但引用地址不同，则不包含。
+        /// </summary>
+        [Fact]
+        public void Contains_Reference_No_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id = 1, Name = "王小明",Age = 10 },
+                new Person(){ Id = 2, Name = "王小庆",Age = 66 },
+                new Person(){ Id = 3, Name = "西门之王",Age = 45 },
+            };
+
+            var current = new Person() { Id = 1, Name = "王小明", Age = 10 };
+            var anyQuery = peoples.Contains(current);
+
+            Assert.False(anyQuery);
+        }
+
+        /// <summary>
+        /// 引用类型，包含：引用地址相同。
+        /// </summary>
+        [Fact]
+        public void Contains_Reference_Yes_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id = 1, Name = "王小明",Age = 10 },
+                new Person(){ Id = 2, Name = "王小庆",Age = 66 },
+                new Person(){ Id = 3, Name = "西门之王",Age = 45 },
+            };
+
+            var queryItem = peoples.First();
+            var isContain = peoples.Contains(queryItem);
+
+            Assert.True(isContain);
+        }
+
+        /// <summary>
+        /// 立即执行
+        /// </summary>
+        [Fact]
+        public void Contains_Immediately_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id = 1, Name = "王小明",Age = 10 },
+                new Person(){ Id = 2, Name = "王小庆",Age = 66 },
+                new Person(){ Id = 3, Name = "西门",Age = 45 },
+            };
+
+            var addItem = new Person() { Id = 4, Name = "张三", Age = 33 };
+
+            //不包含指定项
+            var containQuery = peoples.Contains(addItem);
+
+            //操作后，添加到集合，此时应该包含
+            peoples.Add(addItem);
+
+            //断言：不包含
+            //因为添加前，已经立即执行，保留结果；后面再添加项，不影响以前的执行结果。
+            Assert.False(containQuery);
+        }
+
+        /// <summary>
+        /// 重载方法：使用自定义比较器
+        /// 虽然大部分属性都不一样，但是比较器认为Id属性相同即相等，Contains就认为包含。
+        /// </summary>
+        [Fact]
+        public void Contains_Overload_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id = 1, Name = "王小明",Age = 10 },
+                new Person(){ Id = 2, Name = "东方不败",Age = 66 },
+                new Person(){ Id = 3, Name = "西门吹雪",Age = 45 },
+            };
+
+            var people = new Person() { Id = 3, Name = "小清闲", Age = 88 };
+
+            //自定义比较器：Id相同即为同一个对象
+            PersonEqualId equalId = new PersonEqualId();
+            var anyQuery = peoples.Contains(people, new PersonEqualId());
+
+            //比较器相等，即包含
+            Assert.True(anyQuery);
+        }
+
+        /// <summary>
+        ///  NullReferenceException 异常
+        /// </summary>
+        [Fact]
+        public void Contains_NullReferenceException_Test()
+        {
+            List<Person> peoples = null;
+
+            var people = new Person() { Id = 3, Name = "小清闲", Age = 88 };
+
+            Action anyQueryAction = () => peoples.Contains(people);
+
+            Assert.Throws<NullReferenceException>(anyQueryAction);
+        }
+        #endregion
+    }
+}
diff --git a/LinqStudy.Test/LinqToObject/SequenceEqualTest.cs b/LinqStudy.Test/LinqToObject/SequenceEqualTest.cs
index 981c1ea..49c318a 100644
--- a/LinqStudy.Test/LinqToObject/SequenceEqualTest.cs
+++ b/LinqStudy.Test/LinqToObject/SequenceEqualTest.cs
@@ -159,16 +159,16 @@ namespace LinqStudy.Test.LinqToObject
         [Fact]
         public void SequenceEqual_OverrideEquals_Test()
         {
-            List<Student> sequence1 = new List<Student>()
+            List<StudentOverriteEquals> sequence1 = new List<StudentOverriteEquals>()
             {
-                new Student(){Id=1,Name="张三",Age=20},
-                new Student(){Id=1,Name="李四",Age=20},
+                new StudentOverriteEquals(){Id=1,Name="张三",Age=20},
+                new StudentOverriteEquals(){Id=1,Name="李四",Age=20},
             };
 
-            List<Student> sequence2 = new List<Student>()
+            List<StudentOverriteEquals> sequence2 = new List<StudentOverriteEquals>()
             {
-                new Student(){Id=1,Name="李四",Age=20},
-                new Student(){Id=1,Name="张三",Age=20},
+                new StudentOverriteEquals(){Id=1,Name="李四",Age=20},
+                new StudentOverriteEquals(){Id=1,Name="张三",Age=20},
             };
 
             Assert.True(sequence1.SequenceEqual(sequence2));
@@ -178,7 +178,7 @@ namespace LinqStudy.Test.LinqToObject
         /// 提供自定义比较器
         /// </summary>
         [Fact]
-        public void Test()
+        public void SequenceEqual_CustomEqual_Test()
         {
             List<Person> sequence1 = new List<Person>()
             {
@@ -194,55 +194,5 @@ namespace LinqStudy.Test.LinqToObject
 
             Assert.True(sequence1.SequenceEqual(sequence2, new PersonEqual()));
         }
-
-        /// <summary>
-        /// 自定义Person比较器
-        /// Id相同，则表示同一个对象
-        /// </summary>
-        private class PersonEqual : IEqualityComparer<Person>
-        {
-            bool IEqualityComparer<Person>.Equals(Person x, Person y)
-            {
-                if (x==null || y==null)
-                {
-                    return false;
-                }
-
-                return x.Id == y.Id;
-            }
-
-            int IEqualityComparer<Person>.GetHashCode(Person obj)
-            {
-                return obj.Id.GetHashCode();
-            }
-        }
-
-        /// <summary>
-        /// 重写比较方法
-        /// </summary>
-        private class Student
-        {
-            public int Id { get; set; }
-
-            public string Name { get; set; }
-
-            public int Age { get; set; }
-
-            public override bool Equals(object obj)
-            {
-                var other = obj as Student;
-                if (other == null)
-                {
-                    return false;
-                }
-
-                return Id==other.Id;
-            }
-
-            public override int GetHashCode()
-            {
-                return this.Id.GetHashCode();
-            }
-        }
     }
 }
diff --git a/LinqStudy.Test/LinqToObject/TypeConvertTest.cs b/LinqStudy.Test/LinqToObject/TypeConvertTest.cs
index 8cd6c0b..51f0e9b 100644
--- a/LinqStudy.Test/LinqToObject/TypeConvertTest.cs
+++ b/LinqStudy.Test/LinqToObject/TypeConvertTest.cs
@@ -375,26 +375,6 @@ namespace LinqStudy.Test.LinqToObject
             return dt;
         }
 
-        private Students GetStudents()
-        {
-            Students arrStudent = new Students
-            (
-                new Student[]
-                {
-                    new Student() { Name = "张三", Age = 23 },
-                    new Student() { Name = "李四", Age = 24 },
-                    new Student() { Name = "王五", Age = 45 },
-                    new Student() { Name = "DDD", Age = 21 },
-                    new Student() { Name = "BBB", Age = 14 },
-                    new Student() { Name = "CCC", Age = 3 },
-                    new Student() { Name = "AAA", Age = 22 },
-                    new Student() { Name = "BBB", Age = 55 },
-                    new Student() { Name = "CCC", Age = 43 },
-                }
-            );
-
-            return arrStudent;
-        }
         #endregion
     }
 }
diff --git a/LinqStudy.Test/LinqToObject/WhereTest.cs b/LinqStudy.Test/LinqToObject/WhereTest.cs
index efed245..fc54e67 100644
--- a/LinqStudy.Test/LinqToObject/WhereTest.cs
+++ b/LinqStudy.Test/LinqToObject/WhereTest.cs
@@ -9,10 +9,13 @@ using Xunit;
 namespace LinqStudy.Test.LinqToObject
 {
     /// <summary>
-    /// where操作符：过滤查询条件
+    /// where操作符
     /// </summary>
     public class WhereTest
     {
+        /// <summary>
+        /// where：过滤查询条件
+        /// </summary>
         [Fact]
         public void Where_Test()
         {
@@ -28,8 +31,11 @@ namespace LinqStudy.Test.LinqToObject
             Assert.Equal(2, age);
         }
 
+        /// <summary>
+        /// ArgumentNullException 异常
+        /// </summary>
         [Fact]
-        public void Where_Argm_Test()
+        public void Where_ArgumentNullException_Test()
         {
             List<Person> Persons = null;
 
diff --git a/LinqStudy.Test/LinqToObject/标准查询操作符.md b/LinqStudy.Test/LinqToObject/标准查询操作符.md
new file mode 100644
index 0000000..d8b9020
--- /dev/null
+++ b/LinqStudy.Test/LinqToObject/标准查询操作符.md
@@ -0,0 +1,35 @@
+# 标准查询操作符
+
+## 投影操作符
+
+    Select
+
+### SelectMany
+
+## 限制操作符
+
+### Where
+
+## 排序操作符
+
+### OrderBy
+
+## 联接操作符
+
+## 分组操作符
+
+## 串联操作符
+
+## 聚合操作符
+
+## 生成操作符
+
+## 转换操作符
+
+## 元素操作符
+
+## 相等操作符
+
+## 量词操作符
+
+## 分割操作符
\ No newline at end of file
diff --git a/LinqStudy/Class1.cs b/LinqStudy/Class1.cs
deleted file mode 100644
index d85860f..0000000
--- a/LinqStudy/Class1.cs
+++ /dev/null
@@ -1,8 +0,0 @@
-using System;
-
-namespace LinqStudy
-{
-    public class Class1
-    {
-    }
-}
diff --git a/LinqStudy/Models/Person.cs b/LinqStudy/Models/Person.cs
index 29e01e4..da45934 100644
--- a/LinqStudy/Models/Person.cs
+++ b/LinqStudy/Models/Person.cs
@@ -2,6 +2,9 @@ using System;
 
 namespace LinqStudy
 {
+    /// <summary>
+    /// ��
+    /// </summary>
     public class Person
     {
         public int Id{get;set;}
diff --git a/LinqStudy/Models/PersonEqual.cs b/LinqStudy/Models/PersonEqual.cs
index 9930af9..86237d0 100644
--- a/LinqStudy/Models/PersonEqual.cs
+++ b/LinqStudy/Models/PersonEqual.cs
@@ -5,7 +5,7 @@ using System.Text;
 namespace LinqStudy
 {
     /// <summary>
-    /// Person自定义比较类
+    /// Person自定义比较器
     /// 各属性均同，则相等.有一个Null值则不相等。
     /// </summary>
     public class PersonEqual : IEqualityComparer<Person>
diff --git a/LinqStudy/Models/PersonEqualId.cs b/LinqStudy/Models/PersonEqualId.cs
new file mode 100644
index 0000000..7a7b8c6
--- /dev/null
+++ b/LinqStudy/Models/PersonEqualId.cs
@@ -0,0 +1,28 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace LinqStudy
+{
+    /// <summary>
+    /// Person 自定义ID比较器
+    /// Id相同，则视为相同
+    /// </summary>
+    public class PersonEqualId : IEqualityComparer<Person>
+    {
+        bool IEqualityComparer<Person>.Equals(Person x, Person y)
+        {
+            if (x==null || y==null)
+            {
+                return false;
+            }
+
+            return x.Id == y.Id;
+        }
+
+        int IEqualityComparer<Person>.GetHashCode(Person obj)
+        {
+            return (obj.Id).GetHashCode();
+        }
+    }
+}
diff --git a/LinqStudy/Models/StudentOverriteEquals.cs b/LinqStudy/Models/StudentOverriteEquals.cs
new file mode 100644
index 0000000..86fa9b5
--- /dev/null
+++ b/LinqStudy/Models/StudentOverriteEquals.cs
@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace LinqStudy
+{
+    /// <summary>
+    /// 重写比较方法的学生类
+    /// </summary>
+    public class StudentOverriteEquals
+    {
+        public int Id { get; set; }
+
+        public string Name { get; set; }
+
+        public int Age { get; set; }
+
+        public override bool Equals(object obj)
+        {
+            var other = obj as StudentOverriteEquals;
+            if (other == null)
+            {
+                return false;
+            }
+
+            return Id == other.Id;
+        }
+
+        public override int GetHashCode()
+        {
+            return this.Id.GetHashCode();
+        }
+    }
+}