C#6 String interpolation + short format string bug?

Question

C# 6 introduced string interpolation and a shorter way to specify the format string.

IntPtr ptr = new IntPtr(0xff);

Console.WriteLine(ptr.ToString());      // 255
Console.WriteLine(ptr.ToString("x"));   // ff

Console.WriteLine($"0x{ptr.ToString("x")}"); // 0xff
Console.WriteLine($"0x{ptr:x}"); //0x255

Why the two last lines output a different result ? Am I missing something ?

Try it with DotnetFiddle

As a side note here is the source code of IntPtr ToString() in dotnet core :

public unsafe  String ToString(String format) 
    {
        #if WIN32
            return ((int)m_value).ToString(format, CultureInfo.InvariantCulture);
        #else
            return ((long)m_value).ToString(format, CultureInfo.InvariantCulture);
        #endif
    }

You shouldn't really be comparing with ToString - it'd be more accurate to compare with string.Format("{0:x}", ptr); which also returns 255. — Damien_The_Unbeliever
– Damien_The_Unbeliever, Commented Mar 27, 2020 at 8:24

ProgrammingLlama · Accepted Answer · 2020-03-27 08:32:41Z

7

Your example code:

Console.WriteLine($"0x{ptr:x}");

Is equivalent to its string.Format brother:

Console.WriteLine(string.Format("0x{0:x}", ptr));

When applying your format string "x", string interpolation / string format ultimately reaches this line of code:

IFormattable formattableArg = arg as IFormattable;

Unfortunately, while IntPtr has a custom format ToString() method, it doesn't implement IFormattable, so it's basic .ToString() method is called and the format string is discarded.

See this question for more information

As vasily.sib suggested, you can use $"0x{(int)ptr:x}" instead.

Try my example.

edited Mar 27, 2020 at 8:32

answered Mar 27, 2020 at 8:26

ProgrammingLlama

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3 Comments

KVM Over a year ago

Since IntPtr doesn't implement IFormattable, why does ptr.ToString("x") formats to hexadecimal ?

ProgrammingLlama Over a year ago

@Kvm As I explained, it has a custom ToString method that takes a format string. I even provided a link to it in my answer so that you can read its code.

Ben Voigt Over a year ago

"the format string is discarded" is something that the Framework obviously ought to detect and warn about, preferably at compile time (since interpolated string are already undergoing rather heavy transformation by the compiler), but if not then it ought to spit out some warning at runtime.

fdafadf · Accepted Answer · 2020-03-27 08:39:19Z

0

As documentation says:

https://learn.microsoft.com/en-us/dotnet/standard/base-types/standard-numeric-format-strings

For example, use "X" to produce "ABCDEF", and "x" to produce "abcdef". This format is supported only for integral types.

Pointer is not integral type. For example:

string.Format("{0:x}", ptr)

also returns 255.

answered Mar 27, 2020 at 8:39

fdafadf

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3 Comments

KVM Over a year ago

Since IntPtr is not an integral type, then why does ptr.ToString("x") formats to hexadecimal ?

fdafadf Over a year ago

If you want know more than the documentation, for example you can check an implementation of ToString method of IntPtr class. public unsafe string ToString(string format) { return ((long)m_value).ToString(format, CultureInfo.InvariantCulture); } It uses internally integral type (long).

Ben Voigt Over a year ago

Taking this line of reasoning one step further, "pointer is not numeric type", and so that documentation page is just as inapplicable as "standard date/time format strings".

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