ReSharper claims that my expression is always true? Is it correct or is it a bug?

Question

ReSharper is claiming that my null check against serviceProvider is always true, which is odd. I figured that serviceProvider very well might be null. Am I wrong here, or is this a ReSharper bug?

public override object ProvideValue(IServiceProvider serviceProvider)
{
    switch (Mode)
    {
        case BindingMode.TwoWay:
            throw new InvalidOperationException("Invalid binding mode.");

        case BindingMode.OneWayToSource:
            throw new InvalidOperationException("Invalid binding mode.");

        case BindingMode.OneWay:
        case BindingMode.OneTime:
            break;

        case BindingMode.Default:
            if (serviceProvider != null) // Expression is always true?? O.o
            {
                // Returns something possibly...
            }

            throw new InvalidOperationException("Invalid binding mode.");

        default:
            throw new InvalidOperationException("Unexpected binding mode.");
    }

    return base.ProvideValue(serviceProvider);
}

Update:

I created a console application, and the following code gives a warning as well (from ReSharper) Possible null assignment to entity marked with 'NotNull' attribute.

var binding = new CustomBinding();
binding.ProvideValue(null);

sounds like a R# bug, unless you haven't shown the full code? — Federico Berasategui, Dec 23 '14 at 16:47
@Jonesy: ReSharper warns me if I attempt to pass in null as well. — myermian, Dec 23 '14 at 16:56

score 3 · Accepted Answer · edited May 23 '17 at 12:01

Taken from the documentation at http://msdn.microsoft.com/en-us/library/system.windows.markup.typeextension.providevalue%28v=vs.110%29.aspx

You can pass null for serviceProvider, but only if this TypeExtension instance was established with an initial true type in the constructor rather than a typeName. Otherwise, this markup extension implementation relies on services based on the passed serviceProvider. It must not be null. The serviceProvider is expected to provide a service for IXamlTypeResolver.

Also you can view When can a generic parameter never be null for more information about this.

Apparently, if i am correct, this method has a contract [NotNull] for its parameter because some implementations throw errors upon null arguments.

score -4 · Answer 2 · answered Dec 23 '14 at 16:48

-4

Since ProvideValue is a function that needs IServiceProvider Resharper correctly assumes that if the serviceProvider is null is being checked prior to calling the ProvideValue function thus the espression is always true

answered Dec 23 '14 at 16:48

elasticrash

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Resharper makes no such assumptions, someone could call the method with `ProvideValue(null);` – Ben Robinson Dec 23 '14 at 16:50
I've seen it quite frequently in older versions so Ill stick to my answer in 9.0 though that particular code doesn't evaluate as always true. Kill me Bad judgement :) – elasticrash Dec 23 '14 at 16:54

ReSharper claims that my expression is always true? Is it correct or is it a bug?

2 Answers2