Answer by user2771704 for What is the difference between String and string in C#?

There's a quote on this issue from Daniel Solis' book.

All the predefined types are mapped directly to underlying .NET types. The C# type names (string) are simply aliases for the .NET types (String or System.String), so using the .NET names works fine syntactically, although this is discouraged. Within a C# program, you should use the C# names rather than the .NET names.