"No overload for method 'ToString' takes 1 arguments" in a C# generic method

Question

I am trying to write a C# generic method that accepts nullable decimal and double values and converts them to a string representation.

I am getting the error "No overload for method 'ToString' takes 1 arguments" although I am accessing .Value of the nullable parameter.

Here is my code. What am I doing wrong?

public static string ToThousandSeparated<T>(T? value, string naString = "") where T : struct
{
    if (value.HasValue)
    {
        T val = value.Value;
        return val.ToString("N0");
    }

    return naString;
}

Answer 1

object only defines the method string ToString() (with no parameters). Objects like Int32 define their own string ToString(string) method.

However, there's a useful interface called IFormattable, which provides a string ToString(string, IFormatProvider) method. So you can either constrain yourself to all T which implement IFormattable:

public static string ToThousandSeparated<T>(T? value, string naString = "")
    where T : struct, IFormattable
{
    if (value.HasValue)
    {
        T val = value.Value;
        return val.ToString("N0", null);
    }

    return naString;
}

or accept anything, but test whether it implements IFormattable at runtime:

public static string ToThousandSeparated<T>(T? value, string naString = "")
    where T : struct
{
    if (value.HasValue)
    {
        T val = value.Value;
        return val is IFormattable formattable ? formattable.ToString("N0", null) : val.ToString();
    }

    return naString;
}