运算符“ ”不能应用于类型的操作数<int>和空

Question

I have the below method that accepts BuildAppStat object as parameter.
However it can be null sometimes.

So I am calling it like as :

sourceId: bas.BuildAppStatId ?? null

and getting compile error

operator ' ' cannot be applied to operand of type <int> and null .

How can I fix this?

public PageBuildVrsn EditPageBuildVrsn(string caption, string nameInUse, string description, BuildAppStat bas = null, int pageBuildVrsnTypeId = 1)
{
    PageBuildVrsn pbv = Create(pageBuildVrsnTypeId, caption, nameInUse, description);
    pbv.ParentBuildAppStats = new List<BuildAppStat>();
    pbv.ParentBuildAppStats.Add(
        BasFactory.Create(
            DateTimeOffset.Now,
            true,
            sourceId: bas.BuildAppStatId ?? null
        )
    );

    return pbv;
}

Answer 1

You can only use the null-coalescing operator with a first operand which is a nullable type (either a nullable value type, or any reference type). It sounds like bas.BuildAppStatId is just an int , which means you can't use the null-coalescing operator with it.

If you were trying to guard against bas itself being null, you want:

sourceId: bas == null ? (int?) null : bas.BuildAppStatId,

Answer 2

From ?? Operator (C# Reference) ?? Operator (C# Reference)

The ?? operator is called the null-coalescing operator and is used to define a default value for nullable value types or reference types .

Since your bas.BuildAppStatId is an int , you can't use ?? in here.

Answer 3

BuildAppStatId should be declared as a nullable int.

int?

then if you have trouble with "?? null", use either

... ?? null as int?

or

... ?? default(int?)