Compiler Error CS0260

Missing partial modifier on declaration of type 'type'; another partial declaration of this type exists

This error indicates that more than one class declaration with the same name was found, and at least one but not all of these declarations was declared partial. If your intention is to define a class in several parts, all those parts must be declared with the keyword partial. This error also occurs if you create a new class that accidentally has the same name as a partial class declared elsewhere in the same namespace.

The following sample generates CS0260:

// CS0260.cs
class C  // CS0260
{
}

partial class C
{
}