C# readonly vs Get

Are there any differences between the readonly modifier and get-only properties?

Example:

public class GetOnly
{
    public string MyProp { get; }
}

public class ReadOnly
{
    public readonly string MyProp;
}

Bonus: is there a way to make an interface that works with both? (to use with generics)

public interface ISomething
{
    public string MyProp { get; }
}

public class GetOnly : ISomething
{
    public string MyProp { get; }
}

public class ReadOnly : ISomething // Cannot implement
{
    public readonly string MyProp;
}

Many thanks in advance!