Cannot implicitly convert type

Exactly what it says, EmployeeHistoryList and EmployeeRegBL are two different classes.

public class TestClass
    {
        public void TestMethod()
        {
            System.Collections.Generic.List<TestClass1> testClassList = new List<TestClass1>();
            testClassList.Add(new TestClass1() { TestVariable = 1, TestVariable1 = "TestValue" });
            testClassList.Add(new TestClass1() { TestVariable = 1, TestVariable1 = "TestValue" });
            testClassList.Add(new TestClass1() { TestVariable = 1, TestVariable1 = "TestValue" });

            // You can convert list to any supported type with the same class like this (i.e List<TestClass1> to IEnumerable<TestClass1>)
            System.Collections.Generic.IEnumerable<TestClass1> testClassEnumerable = testClassList.AsEnumerable();

            // You can't convert list to any supported type with other class like this (i.e List<TestClass1> to IEnumerable<TestClass2>)
            //System.Collections.Generic.IEnumerable<TestClass2> testClassEnumerable2 = testClassList.AsEnumerable();

            //you can Convert like this 'System.Collections.Generic.List<SFM.Models.BL.EmployeeHistoryList>' to 'System.Collections.Generic.IEnumerable<SFM.Models.BL.EmployeeHistoryList>'
            //you can Convert like this 'System.Collections.Generic.List<SFM.Models.BL.EmployeeRegBL>' to 'System.Collections.Generic.IEnumerable<SFM.Models.BL.EmployeeRegBL>'
            //you can't Convert like this 'System.Collections.Generic.List<SFM.Models.BL.EmployeeHistoryList>' to 'System.Collections.Generic.IEnumerable<SFM.Models.BL.EmployeeRegBL>'
        }
    }

    public class TestClass1
    {
        public int TestVariable { get; set; }
        public string TestVariable1 { get; set; }
    }

    public class TestClass2
    {
        public int TestVariable { get; set; }
        public string TestVariable1 { get; set; }
    }

EmployeeHistoryList and EmployeeRegBL are not matching types

You can use .toArray

System.Collections.Generic.List<int> list = new System.Collections.Generic.List<int>( );
list.Add( 1 );
System.Collections.Generic.IEnumerable<int> enumer = list.ToArray( );

Since 'an explicit conversion exists' you need to cast the variable to System.Collections.Generic.IEnumerable<sfm.models.bl.employeeregbl></sfm.models.bl.employeeregbl>

There is also IEnumerable.Cast<TResult>() extension method that will cast the type of all items in collection (if such cast is available).