How can I resolve CS0266 when converting DateTime? to non-nullable DateTime using NotNullIfNotNullAttribute?

The issue isn't with the NotNullIfNotNullAttribute—it doesn’t allow an implicit conversion of DateTime? (nullable) to DateTime (non‑nullable). When calling the helper with a non‑nullable DateTime, you’re really calling the overload that converts it to DateTime? and then trying to assign it to a non‑nullable variable. Since a nullable type might not have a value, C# requires you to explicitly extract the value (or handle the null case).

You can fix this in one of the following ways:

Option 1: Use .Value When You’re Sure It’s Not Null

If you know the date passed isn’t null (for example, because you started with a non‑nullable DateTime), then you can write:

CSHARPCopied
public static void Ex2()
{
    DateTime dt0 = DateTime.Now;
    DateTime dt1 = Utils.ToUtc(dt0)!.Value; // using .Value explicitly
}

The null-forgiving operator (!) tells the compiler that the result isn’t null.

Option 2: Provide an Overload for Non‑Nullable DateTime

Alternatively, you can add an overload that accepts a non‑nullable DateTime and returns a non‑nullable DateTime. For example:

CSHARPCopied
public static class Utils
{
    [return: NotNullIfNotNull(nameof(dt))]
    public static DateTime? ToUtc(DateTime? dt) => dt?.Kind switch
    {
        null => null,
        DateTimeKind.Unspecified => DateTime.SpecifyKind(dt.Value, DateTimeKind.Utc),
        DateTimeKind.Utc => dt,
        DateTimeKind.Local => dt.Value.ToUniversalTime()
    };

    public static DateTime ToUtc(DateTime dt)
        => ToUtc((DateTime?)dt)!.Value;
}

Now, a call like this works without an explicit cast:

CSHARPCopied
public static void Ex2()
{
    DateTime dt0 = DateTime.Now;
    DateTime dt1 = Utils.ToUtc(dt0); // Now returns non-nullable DateTime
}

Summary

The NotNullIfNotNullAttribute only affects static analysis for nullability but doesn’t change the type conversion rules. In order to perform a conversion from DateTime? to DateTime, you must either handle the possibility of null or provide an overload that guarantees a non‑nullable result.