What is the difference between String and string in C#?

In C#, string and String are functionally identical but differ in their usage conventions and syntactic roles. Here’s a detailed breakdown:

1. Technical Equivalence

string is a C# keyword and an alias for System.String, a class in the .NET Framework.
String is the full name of the .NET class (System.String).

They refer to the exsame type and are interchangeable in most contexts. The compiler treats them identically.

2. Key Differences

Aspect	`string` (Keyword)	`String` (Class Name)
Syntax Role	C# language keyword (like `int`, `bool`).	Full class name from the `System` namespace.
Usage Convention	Preferred for variable declarations.	Used for static method calls (e.g., `String.IsNullOrEmpty()`).
Namespace	Does not require `using System;`.	Requires `using System;` (or `System.String`).

3. When to Use Which

Use `string` For:

Variable declarations and parameters.
General string operations (e.g., concatenation, interpolation).

string name = "Alice";          // ✅ Preferred
string message = $"Hello, {name}!";

Use `String` For:

Calling static methods of the String class.
Emphasizing type semantics (e.g., reflection, generics).

if (String.IsNullOrEmpty(name)) // ✅ Preferred for static methods
{
    Console.WriteLine("Name is empty.");
}

// Type checking
Type type = typeof(String);     // Same as typeof(string)

4. Examples of Interchangeability

Variable Declaration (Both Work)

string s1 = "Hello";  // ✅ Conventional
String s2 = "World";  // ⚠️ Valid but less common

Static Methods (Both Work)

bool isEmpty1 = string.IsNullOrEmpty(s1); // ✅ Works but unconventional
bool isEmpty2 = String.IsNullOrEmpty(s2); // ✅ Preferred

Type Comparison

Console.WriteLine(s1.GetType() == typeof(string));  // True
Console.WriteLine(s2.GetType() == typeof(String));  // True

5. Edge Cases

Namespace Conflicts: If you define a custom String class, String refers to your class, while string/System.String refers to the .NET type.

  using System;

  namespace MyApp 
  {
      class String { } // ❗ Avoid this!

      class Program 
      {
          static void Main() 
          {
              String s = new String(); // Refers to MyApp.String
              System.String s2 = "Hello"; // Explicit .NET String
              string s3 = "World"; // Refers to System.String
          }
      }
  }

No using System; Directive:

  // Without "using System;":
  string s1 = "Works";        // ✅ No namespace needed
  System.String s2 = "Works"; // ✅ Fully qualified
  String s3 = "Fails";        // ❌ Compiler error (unless in scope)

6. Best Practices

Use string for variables, parameters, and local declarations.
Use String when invoking static methods (e.g., String.Format(), String.Compare()).
Maintain consistency with your team’s coding standards.

Summary

string is idiomatic in C# and aligns with primitive-type keywords.
String explicitly references the .NET class and is used for static methods.
Both are technically identical—choose based on convention and readability!