C# - Assembly GetType is everytime null

I try to call a Method in a DLL File without add the DLL to the References only throw the File. But I cant get the Type (MainClass) of the DLL. I know the Namespace and the Class where the Method is in. Here my Code:

private Type GetMainClass()
{
        return assembly.GetType(GetCombineContents().MainClass);
}

The Method returns every time null. In Variable assembly contains the DLL Assembly