Restrict object to several types

Question

I need to hold an object in class that is restricted to be either a string or a double. How to do it stylistically right? I came up with idea to create a enum with possible types and two methods for each type to set the field.

public class Operand
{
    /// <summary> Possible types of operand </summary>
    public enum TYPE
    {
        /// <summary> Number </summary>
        Numeric,
        /// <summary> Parameter </summary>
        Parameter,
        /// <summary> Invalid </summary>
        None
    }

    /// <summary> Type of operand </summary>
    public TYPE Type { private set; get; } = TYPE.None;

    /// <summary> Value of operand: double or string </summary>
    public object Value { private set; get; } = null;

    /// <summary> Set the parametric operand </summary>
    public void Set(string value)
    {
        Value = value;
        Type = TYPE.Parameter;
    }

    /// <summary> Set the numeric operand </summary>
    public void Set(double value)
    {
        Value = value;
        Type = TYPE.Numeric;
    }
}

Answer 1

You can set value with one method like this:

public void Set(object value)
{
    if (value is String)
    {
        // Type string
    }
    else if (value is Double)
    {
        // type double
    }
}