C#8 nullable : string.IsNullOrEmpty is not understood by compiler as helping for guarding against null

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I am using C# 8 with .NET framework 4.8

I'm currently guarding against a potential string that can be null with IsNullOrWhitespace (same problem with IsNullOrEmpty) , but the compiler is still complaining :

public MyImage? LoadImage(string? filename)
{
    if (string.IsNullOrWhiteSpace(filename))
    {
        return null;
    }
    return OtherMethod(filename); // here : warning from Visual Studio
}

// signature of other method :
public MyImage OtherMethod(string filepath);

Hint / warning from VS : 'filename' may be null here.

currently, I have workarounds to make the compiler understand :

  • use null forgiving operator filename!
  • disable warning by using #pragma warning disable CS8604 // Possible null reference argument.
  • add another check for null if(string == null || string.IsNullOrWhitespace(filename))

But none of the seems satisfactory, mainly because I'll need to repeat the workaround for each call to IsNullOrEmpty.

Is there any other way to tell the compiler that IsNullOrEmpty effectively guards against null ?

c#-8.0 nullable-reference-types string
1 Answers

If you don't have .net standard 2.1 or .net core 3, the IsNullOrEmpty is not nullable ready. So, I would create an extension method for this:

#nullable enable
public static class StringExt {
    public static bool IsNullOrEmpty([NotNullWhen(false)] this string? data) {
        return string.IsNullOrEmpty(data);
    }
}
#nullable restore

You also need to activate NotNullWhen attribute like this:

namespace System.Diagnostics.CodeAnalysis
{
    [AttributeUsage(AttributeTargets.Parameter)]
    public sealed class NotNullWhenAttribute : Attribute {
        public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
        public bool ReturnValue { get; }
    }
}
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