Description

When an interface is never implemented, all casts to this interface are expected to return null, which may lead to dead code that could be detected.

Sample repro

[UsedImplicitly(ImplicitUseTargetFlags.WithMembers)]
public interface IUsed
{
    string Value { get; }
}

internal interface INotImplemented : IUsed
{
}

[UsedImplicitly(ImplicitUseTargetFlags.WithMembers)]
public class Base
{
    public Base(IUsed value)
    {
        Value = value;
    }
    public IUsed Value { get; }
}

internal sealed class DeadClass : Base
{
    public DeadClass(INotImplemented value)
        : base(value)
    {
        NewValue = value;
    }
    public INotImplemented NewValue { get; }
}

[UsedImplicitly(ImplicitUseTargetFlags.WithMembers)]
public sealed class OtherCode
{
    public Base GetBase(IUsed data)
    {
        var alwaysNull = data as INotImplemented;
        if (alwaysNull != null)
            return new DeadClass(alwaysNull);
        return new Base(data);
    }
}

Expected result

I expect these code inspections to pop up:

• alwaysNull is always null
  
• DeadClass is not used
  
• INotImplemented is never implemented

Actual result

Nothing
But I can safely delete INotImplemented, DeadClass and the 3 first lines of GetBase.

Versions

Resharper 2017.3.5