What is the difference between String.Format and string.Format (and other static members of primitive data types)?

There's no difference, these are type aliases in C# for .Net framework types, you're calling the same method underneath.

For example:

• int is an alias for System.Int32
• string is an alias for System.String

You can find a complete list of these aliases on MSDN here.

Those are not related primitive data types. They are simply shorthands available in C#. string aliases System.String and int aliases System.Int32. Calls to int.MaxValue are calls to Int32.MaxValue. C# just allows you to type it in shorthand similar to what you would type if you were in another C-like language.

Most of the answers have it in general. HOWEVER, there are some cases in which the alias is required. Off the top of my head:

public enum MyEnum:Byte {...} //will not compile

public enum MyEnum:byte {...} //correct

There are a couple other places where you must use the alias; besides that, it's mostly style. The following two rules are generally accepted, perhaps the first more than the second:

• Use the alias (lowercase keyword) for all usages of that define a variable or member type (declaration, parameters, casting, generic type closure)
• Use the type name (PascalCased class identifier) for all usages of the type in static context (Calling static methods such as parsers or string manipulation methods, or static properties like MinValue/MaxValue).