A function to convert null to string

static string NullToString( object Value )
{

    // Value.ToString() allows for Value being DBNull, but will also convert int, double, etc.
    return Value == null ? "" : Value.ToString();

    // If this is not what you want then this form may suit you better, handles 'Null' and DBNull otherwise tries a straight cast
    // which will throw if Value isn't actually a string object.
    //return Value == null || Value == DBNull.Value ? "" : (string)Value;


}

Convert.ToString(object) converts to string. If the object is null, Convert.ToString converts it to an empty string.

Calling .ToString() on an object with a null value throws a System.NullReferenceException.

EDIT:

Two exceptions to the rules:

1) ConvertToString(string) on a null string will always return null.

2) ToString(Nullable<T>) on a null value will return "" .

Code Sample:

// 1) Objects:

object obj = null;

//string valX1 = obj.ToString();           // throws System.NullReferenceException !!!
string val1 = Convert.ToString(obj);    

Console.WriteLine(val1 == ""); // True
Console.WriteLine(val1 == null); // False


// 2) Strings

String str = null;
//string valX2 = str.ToString();    // throws System.NullReferenceException !!!
string val2 = Convert.ToString(str); 

Console.WriteLine(val2 == ""); // False
Console.WriteLine(val2 == null); // True            


// 3) Nullable types:

long? num = null;
string val3 = num.ToString();  // ok, no error

Console.WriteLine(num == null); // True
Console.WriteLine(val3 == "");  // True
Console.WriteLine(val3 == null); // False 

val3 = Convert.ToString(num);  

Console.WriteLine(num == null);  // True
Console.WriteLine(val3 == "");  // True
Console.WriteLine(val3 == null); // False

When you get a NULL value from a database, the value returned is DBNull.Value on which case, you can simply call .ToString() and it will return ""

Example:

 reader["Column"].ToString() 

Gets you "" if the value returned is DBNull.Value

If the scenario is not always a database, then I'd go for an Extension method:

public static class Extensions
{

    public static string EmptyIfNull(this object value)
    {
        if (value == null)
            return "";
        return value.ToString();
    }
}

Usage:

string someVar = null; 
someVar.EmptyIfNull();