diff --git a/LinqStudy.Test/LinqToObject/ElementTest.cs b/LinqStudy.Test/LinqToObject/ElementTest.cs
new file mode 100644
index 0000000..3918a6a
--- /dev/null
+++ b/LinqStudy.Test/LinqToObject/ElementTest.cs
@@ -0,0 +1,455 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+using Xunit;
+
+namespace LinqStudy.Test.LinqToObject
+{
+    /// <summary>
+    /// 元素操作
+    /// </summary>
+    public class ElementTest
+    {
+        #region First
+        /// <summary>
+        /// First：获取第一个元素，延迟执行。
+        /// </summary>
+        [Fact]
+        public void First_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            var queryFrist = peoples.First();
+            var firstItemName = queryFrist.Name;
+
+            Assert.Equal("wanggaofeng", firstItemName);
+        }
+
+        [Fact]
+        public void First_where_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 queryFrist = peoples.First(q => q.Age > 32);
+            var firstItemName = queryFrist.Name;
+
+            Assert.Equal("wanggaofeng", firstItemName);
+        }
+
+        [Fact]
+        public void First_ArgumentNullException_Test()
+        {
+            List<Person> peoples = null;
+
+            Action queryFirstAct = () => peoples.First();
+
+            Assert.Throws<ArgumentNullException>(queryFirstAct);
+        }
+
+        /// <summary>
+        /// 查不到数据时，异常。
+        /// </summary>
+        [Fact]
+        public void First_InvalidOperationException_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            Action queryFrist =() => peoples.First(q => q.Age > 100);
+            
+
+            Assert.Throws<InvalidOperationException>(queryFrist);
+        }
+        #endregion
+
+        #region FirstOrDefault
+        /// <summary>
+        /// FirstOrDefault：获取第一个元素或默认值，延迟执行。
+        /// </summary>
+        [Fact]
+        public void FirstOrDefault_Frist()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            var queryFrist = peoples.FirstOrDefault();
+            var firstItemName = queryFrist.Name;
+
+            Assert.Equal("wanggaofeng", firstItemName);
+        }
+
+        [Fact]
+        public void FirstOrDefault_Default()
+        {
+            List<Person> peoples = new List<Person>();
+
+            var queryFrist = peoples.FirstOrDefault();
+            var firstItem = queryFrist;
+
+            Assert.Null(firstItem);
+        }
+
+        [Fact]
+        public void FirstOrDefault_Where_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 queryFrist = peoples.FirstOrDefault(q => q.Age > 32);
+            var firstItemName = queryFrist.Name;
+
+            Assert.Equal("wanggaofeng", firstItemName);
+        }
+
+        [Fact]
+        public void FirstOrDefault_ArgumentNullException_Test()
+        {
+            List<Person> peoples = null;
+
+            Action queryFirstAct = () => peoples.First();
+
+            Assert.Throws<ArgumentNullException>(queryFirstAct);
+        }
+
+        /// <summary>
+        /// 查不到数据时，不异常。
+        /// </summary>
+        [Fact]
+        public void FirstOrDefault_No_InvalidOperationException()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            var defaultItem = peoples.FirstOrDefault(q => q.Age > 100);
+
+            Assert.Null(defaultItem);
+        }
+        #endregion
+
+        #region Last
+        /// <summary>
+        /// First：获取最后一个元素，延迟执行。
+        /// </summary>
+        [Fact]
+        public void Last_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            var lastItem = peoples.Last();
+            var lastItemName = lastItem.Name;
+
+            Assert.Equal("lishi", lastItemName);
+        }
+
+        [Fact]
+        public void Last_where_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 lastItem = peoples.Last(q => q.Age > 32);
+            var lastItemName = lastItem.Name;
+
+            Assert.Equal("wangwu", lastItemName);
+        }
+
+        [Fact]
+        public void Last_ArgumentNullException_Test()
+        {
+            List<Person> peoples = null;
+
+            Action queryAction = () => peoples.First();
+
+            Assert.Throws<ArgumentNullException>(queryAction);
+        }
+
+        /// <summary>
+        /// 查不到数据时，异常。
+        /// </summary>
+        [Fact]
+        public void Last_InvalidOperationException_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            Action lastAction = () => peoples.Last(q => q.Age > 100);
+
+            Assert.Throws<InvalidOperationException>(lastAction);
+        }
+        #endregion
+
+        #region LastOrDefault
+        /// <summary>
+        /// LastOrDefault：获取最后一个元素或默认值，延迟执行。
+        /// </summary>
+        [Fact]
+        public void LastOrDefault_Last()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            var queryItem = peoples.LastOrDefault();
+            var lastItemName = queryItem.Name;
+
+            Assert.Equal("lishi", lastItemName);
+        }
+
+        [Fact]
+        public void LastOrDefault_Default()
+        {
+            List<Person> peoples = new List<Person>();
+
+            var queryItem = peoples.LastOrDefault();
+
+            Assert.Null(queryItem);
+        }
+
+        [Fact]
+        public void LastOrDefault_Where_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.LastOrDefault(q => q.Age > 32);
+            var lastItemName = queryItem.Name;
+
+            Assert.Equal("wangwu", lastItemName);
+        }
+
+        [Fact]
+        public void LastOrDefault_ArgumentNullException_Test()
+        {
+            List<Person> peoples = null;
+
+            Action queryLastItme = () => peoples.LastOrDefault();
+
+            Assert.Throws<ArgumentNullException>(queryLastItme);
+        }
+
+        /// <summary>
+        /// 查不到数据时，不异常。
+        /// </summary>
+        [Fact]
+        public void LastOrDefault_No_InvalidOperationException()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            var defaultItem = peoples.LastOrDefault(q => q.Age > 100);
+
+            Assert.Null(defaultItem);
+        }
+        #endregion
+
+        #region Single
+        /// <summary>
+        /// Single：获取单个元素，延迟执行。
+        /// </summary>
+        [Fact]
+        public void Single_Test()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33}
+            };
+
+            var queryItem = peoples.Single();
+            var singleName = queryItem.Name;
+
+            Assert.Equal("wanggaofeng", singleName);
+        }
+
+        [Fact]
+        public void Single_where_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 singleQuery = peoples.Single(q => q.Age > 33);
+            var singleItemName = singleQuery.Name;
+
+            Assert.Equal("wangwu", singleItemName);
+        }
+
+        [Fact]
+        public void Single_ArgumentNullException_Test()
+        {
+            List<Person> peoples = null;
+
+            Action singleQueryAction = () => peoples.Single();
+
+            Assert.Throws<ArgumentNullException>(singleQueryAction);
+        }
+
+        /// <summary>
+        /// 查不到数据时，异常。
+        /// </summary>
+        [Fact]
+        public void Single_QueryEmpty_InvalidOperationException()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            Action singleQueryAction = () => peoples.Single(q => q.Age > 100);
+
+
+            Assert.Throws<InvalidOperationException>(singleQueryAction);
+        }
+
+        /// <summary>
+        /// 查询到多条记录，异常
+        /// </summary>
+        [Fact]
+        public void Single_QueryManyItem_InvalidOperationException()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            Action singleQueryAction = () => peoples.Single(q => q.Age > 0);
+
+
+            Assert.Throws<InvalidOperationException>(singleQueryAction);
+        }
+        #endregion
+
+        #region SingleOrDefault
+        /// <summary>
+        /// SingleOrDefault：获取单个元素或默认值，延迟执行。
+        /// </summary>
+        [Fact]
+        public void SingleOrDefault_Last()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33}
+            };
+
+            var queryItem = peoples.SingleOrDefault();
+            var singleItemName = queryItem.Name;
+
+            Assert.Equal("wanggaofeng", singleItemName);
+        }
+
+        [Fact]
+        public void SingleOrDefault_Default()
+        {
+            List<Person> peoples = new List<Person>();
+
+            var queryItem = peoples.SingleOrDefault();
+
+            Assert.Null(queryItem);
+        }
+
+        [Fact]
+        public void SingleOrDefault_Where_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.SingleOrDefault(q => q.Id > 2);
+            var sigleItemName = queryItem.Name;
+
+            Assert.Equal("wangwu", sigleItemName);
+        }
+
+        [Fact]
+        public void SingleOrDefault_ArgumentNullException()
+        {
+            List<Person> peoples = null;
+
+            Action queryAction = () => peoples.LastOrDefault();
+
+            Assert.Throws<ArgumentNullException>(queryAction);
+        }
+
+        /// <summary>
+        /// 查询不到数据，不异常
+        /// </summary>
+        [Fact]
+        public void SingleOrDefault_QueryEmpty_NoArgumentNullException()
+        {
+            List<Person> peoples = new List<Person>();
+
+            var queryItme = peoples.SingleOrDefault();
+
+            Assert.Null(queryItme);
+        }
+
+        /// <summary>
+        /// 查到多条数据时，异常。
+        /// </summary>
+        [Fact]
+        public void SingleOrDefault_No_InvalidOperationException()
+        {
+            List<Person> peoples = new List<Person>()
+            {
+                new Person(){ Id=1, Name="wanggaofeng", Age=33},
+                new Person(){ Id=2, Name="lishi", Age=32},
+            };
+
+            Action singleOrDefalutAction =() => peoples.SingleOrDefault(q => q.Age > 0);
+
+            Assert.Throws<InvalidOperationException>(singleOrDefalutAction);
+        }
+        #endregion
+    }
+}
diff --git a/LinqStudy.Test/LinqToObject/TypeConvertTest.cs b/LinqStudy.Test/LinqToObject/TypeConvertTest.cs
index 349e366..8cd6c0b 100644
--- a/LinqStudy.Test/LinqToObject/TypeConvertTest.cs
+++ b/LinqStudy.Test/LinqToObject/TypeConvertTest.cs
@@ -184,8 +184,6 @@ namespace LinqStudy.Test.LinqToObject
                 new Person() {Id=2, Name="zhangsan",Age = 25 },
             };
 
-            IEqualityComparer<Person> equal = new PersonEqual();
-
             var dic = persons.ToDictionary<Person, int>(p => p.Id, EqualityComparer<int>.Default);
 
             Assert.Equal("gaofeng", dic[1].Name);
diff --git a/LinqStudy.Test/LinqToObject/WhereTest.cs b/LinqStudy.Test/LinqToObject/WhereTest.cs
index 62703c9..a42d55f 100644
--- a/LinqStudy.Test/LinqToObject/WhereTest.cs
+++ b/LinqStudy.Test/LinqToObject/WhereTest.cs
@@ -1,4 +1,5 @@
-using System;
+<<<<<<< .mine
+using System;
 using System.Collections.Generic;
 using System.Text;
 using System.Linq;
@@ -14,6 +15,24 @@ namespace LinqStudy.Test.LinqToObject
     public class WhereTest
     {
 
+=======
+using System;
+using System.Linq;
+using System.Collections;
+using System.Collections.Generic;
+
+using Xunit;
+
+namespace LinqStudy.Test.LinqToObject
+{
+
+    /// <summary>
+    /// 选件筛选
+    /// </summary>
+    public class WhereTest
+    {
+        /// <summary>
+>>>>>>> .theirs
         [Fact]
         public void Where_Test()
         {