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Compiler Error CS1100

Method 'name' has a parameter modifier 'this' which is not on the first parameter.

The this modifier is allowed only on the first parameter of a method, which indicates to the compiler that the method is an extension method.

To correct this error

  • Remove the this modifier from all except the first parameter of the method.

Example

The following code generates CS1100 because a this parameter is modifying the second parameter:

// cs1100.cs
static class Test
{
    static void ExtMethod(int i, this Test c) // CS1100
    {
    }
}

See Also

Reference

Extension Methods (C# Programming Guide)