C#: What is the difference between ref and out parameters in C#?

ref — must be initialized before passing. out — must be assigned inside method.

ref parameter:

• Must be initialized before call.
• Can be read and written.
• Two-way data flow.

1void Increment(ref int x) {
2    x++;  // Read and modify
3}
4
5int a = 5;
6Increment(ref a);  // a must be initialized
7Console.WriteLine(a);  // 6

out parameter:

• Need not be initialized.
• Must be assigned inside method.
• One-way output.

1bool TryParse(string s, out int result) {
2    result = int.Parse(s);  // Must assign
3    return true;
4}
5
6int number;  // No initialization needed
7bool success = TryParse("123", out number);

Key differences:

• ref — initialized before, can read.
• out — not initialized, must assign.

Use cases:

• ref — modify existing value.
• out — return multiple values.