C# public variable as writeable inside the class but readonly outside the class

Solution 1:

Don't use a field - use a property:

class Foo
{
    public string Bar { get; private set; }
}

In this example Foo.Bar is readable everywhere and writable only by members of Foo itself.

As a side note, this example is using a C# feature introduced in version 3 called automatically implemented properties. This is syntactical sugar that the compiler will transform into a regular property that has a private backing field like this:

class Foo
{
    [CompilerGenerated]
    private string <Bar>k__BackingField;

    public string Bar
    {
        [CompilerGenerated]
        get
        {
            return this.<Bar>k__BackingField;
        }
        [CompilerGenerated]
        private set
        {
            this.<Bar>k__BackingField = value;
        }
    }
}

Solution 2:

public class Foo
{
  public string Bar { get; private set; } 
}

Solution 3:

You have to use a property for this. If you are fine with an automatic getter/setter implementation, this will work:

public string SomeProperty { get; private set; }

Note that you should not expose fields as public anyway, except in some limited circumstances. Use a property instead.

Solution 4:

Sure. Make it a property, and make the setter private:

public Int32 SomeVariable { get; private set; }

Then to set it (from within some method in the class):

SomeVariable = 5;