LGTM.com - false positive C# if `is` followed by `as`

[tznind]

Warning for 'value might be null' because upstream path potentially contains a null. Caused by use of as keyword but preceeding is keyword should make it impossible.

if (value is string)
	Validate(fi, dicomFile, dicomItem, value as string); //<- lgtm considers that 'value' could be null because of as

https://lgtm.com/projects/g/SMI/SmiServices/snapshot/673082cacc04b4597e96c7461c47096b94d08a45/files/src/microservices/Microservices.IsIdentifiable/Runners/DicomFileRunner.cs?sort=name&dir=ASC&mode=heatmap#x75953d6da49aa143:1

Admittedly the code itself could be better. It should probably use a direct cast instead of as or use the declaration feature of C# 7.0 (i.e. if(value is string s))

Unit test:

        [Test]
        public void TestNull()
        {
            string a = null;

            Assert.IsFalse(a is string);
            a = "yayy";
            Assert.IsTrue(a is string);
        }

[hvitved]

Hi Thomas

Thanks for reporting this. I agree that it is a false positive, and I have created PR to record it here.