For what purposes are fields with the readonly modifier used in C#?

A field with the readonly modifier in C# can only be initialized at the point of declaration or in the class constructor. After initialization, its value cannot be changed.

Uses:

1. Ensuring immutability: Guarantees that the field's value will not be accidentally changed after object creation. This increases code reliability and simplifies debugging.
2. Constants for each instance: Unlike static constants (const), readonly fields can have different values for different class instances.
3. Initialization in constructor: Allows setting a final value for the field based on constructor logic, which is not possible for const fields (they must be initialized with a literal or constant at compile time).
4. Performance improvement when working with structures: For structures, readonly fields can improve performance because the compiler can make certain optimizations knowing that the value will not change.

Example:

public class MyClass
{
    public readonly int ImmutableValue; // Can be initialized here or in the constructor
    public readonly string Name;

    // Constructor for assigning values to readonly fields
    public MyClass(int value, string name)
    {
        ImmutableValue = value;
        Name = name;
    }

    // This method will cause a compilation error because it tries to modify a readonly field
    // public void TryChangeValue(int newValue)
    // {
    //     ImmutableValue = newValue; // Compilation error
    // }
}