OOP & concepts

string vs String in C#: is there any difference?

OOP & concepts · By the Postback Notes editorial team · Published 2026-08-09

There is no difference in behaviour: string is a C# keyword that is an alias for the .NET type System.String. Both spellings compile to exactly the same thing. The choice is a matter of style, with one or two technical corners.

Same type

string a = "hello";
String b = "hello";             // needs 'using System;' (or global usings)
System.String c = "hello";

Console.WriteLine(a.GetType() == typeof(String));   // True
Console.WriteLine(typeof(string) == typeof(System.String)); // True

Static members are reachable through either name: string.IsNullOrEmpty(x) and String.IsNullOrEmpty(x) are the same call.

Where the spellings differ

• Scope. string is a keyword and always means System.String. String is an ordinary type name, resolved through using directives; in theory a different type called String in your namespace would take precedence.
• Without using System;, String does not compile, string does. Modern SDK projects add implicit global usings, so this rarely bites.
• Keyword contexts. Only the keyword works where the language expects one, for example in some older syntaxes such as an enum's underlying type (enum E : int requires the keyword for int).

The other aliases

The .NET name is useful in method names that describe a type, as the framework itself does: Convert.ToInt32, reader.GetInt64, not ToInt.

Which to use?

Microsoft's C# coding conventions recommend the language keywords (string, int) rather than the runtime type names for declarations and member access, and the default code-style rule IDE0049 in Visual Studio suggests the same. Some teams prefer String.Format for static calls; what matters is consistency within a code base, enforced by an .editorconfig setting.

A more substantial question about strings is mutability: see string vs StringBuilder.

A note on VB.NET

Visual Basic has the same arrangement: String (with a capital letter, as VB keywords are written) is a keyword alias for System.String, just as Integer maps to System.Int32. Code that mixes C# and VB.NET projects therefore uses one type throughout; only the spelling differs between the languages. In both, string comparison rules (ordinal, culture-aware, case-insensitive) matter far more in practice than which alias you type: pass a StringComparison value to Equals, StartsWith and IndexOf to make the intent explicit.

Official documentation

• Microsoft Learn: Built-in types (C# reference), The string type
• Microsoft Learn: C# coding conventions, IDE0049