Compiler Error CS1737

Optional parameters must appear after all required parameters

The compiler does not support optional parameters being declared before required parameters. All optional parameters must be after all required parameters.

Example

The following sample generates CS1737:

// CS1737.cs (7,45)
class C
{
    static void F(object? x)
    {
        G(y: x);
    }
    static void G(object? x = null, object y)
    {
    }
}

To correct this error

The signature for this method may be changed without effecting existing code that calls the method because a value for the optional parameter has not been used. For example:

// CS1737.cs (7,45)
class C
{
    static void F(object? x)
    {
        G(y: x);
    }
    static void G(object y, object? x = null)
    {
    }
}