Why is this an issue?

Calling GetType() on a nullable value type object returns the underlying value type. Therefore, comparing the returned Type object to typeof(Nullable<SomeType>) will either throw an NullReferenceException or the result will always be true or false and can be known at compile time.

How to fix it

Code examples

Noncompliant code example

void DoChecks<T>(Nullable<T> value) where T : struct
{
    bool areEqual = value.GetType() == typeof(Nullable<int>); // Noncompliant: always false
    bool areNotEqual = value.GetType() != typeof(Nullable<int>); // Noncompliant: always true

    Nullable<int> nullable = null;
    bool nullComparison = nullable.GetType() != typeof(Nullable<int>); // Noncompliant: throws NullReferenceException
}

Compliant solution

void DoChecks<T>(Nullable<T> value) where T : struct
{
    bool areEqual = value.GetType() == typeof(int); // Compliant: can be true or false
    bool areNotEqual = value.GetType() != typeof(int); // Compliant: can be true or false

    Nullable<int> nullable = null;
    bool nullComparison = nullable is not null && nullable.GetType() == typeof(int); // Compliant: does not throw NullReferenceException
}

Resources

Documentation