The **`readonly` keyword** is a C# modifier used to limit access to all the data members of a struct.  If the `readonly` modifier is used in the declaration of a struct, then:

1. The members of the struct are read-only.
2. None of the members can have *setters*.
2. A parameterized constructor is used to initialize the data members.
3. The struct is immutable.
4. The `this` variable cannot be changed in any other method except the constructor.

>Remember: `this` is a reference to the current object in a class/struct.

# Code
When you try to run the code below, a few errors will pop-up.

>Rememeber to use a C# version that's 7.2 or above. 

public readonly struct Marvel
{
    // You can have getter methods but not setter methods.
    public string CharacterName { get;set; }//*ERROR*
    // `this` variable can be changed in 
    // constructor
    public Marvel(string name)
    {
        this.CharacterName = name;// *CORRECT*
    }
    // `this` variable cannot be changed except 
    // for constructor  
    public void SwitchCharacter(Marvel M)
    {
        this.CharacterName = M.CharacterName;// *ERROR*
    }
}

Take a look below for the correct implementation of the code above:

public readonly struct Marvel
{
    // No setter method included.
    public string CharacterName { get; }//*Correct*
    // `this` variable can be changed in 
    // constructor
    public Marvel(string name)
    {
        this.CharacterName = name;//*CORRECT*
    }
}

What are readonly structs in C#?

Readonly structs in C# ensure all struct members are immutable, utilizing parameterized constructors.