CODEWITHSUNDEEP
c#nullout
MatthewHagemann
MatthewHagemann

Reputation: 1195

C# Optional Out Parameter

Is there any way to make an out parameter of a method optional? I have a method that needs several parameters to return null most of the time, but sometimes they will have a value. I am trying to find out if there is a better way to accomplish this than by setting the values to null first, or setting them to null after the if/else. This may be the only way to do it, but it would be nice if there were optional out parameters or something equivalent. Here is the code:

    private static void GetInitInfo(string initLine, string ncPointType, out double? relinquishDefault, out bool? ignoreRelinquishDefault, out bool? closedForStart, out bool? adjustDisabled)
    {
        relinquishDefault = null;
        ignoreRelinquishDefault = null;
        closedForStart = null;
        adjustDisabled = null;

        switch (ncPointType)
        {
            case "MSD":
                adjustDisabled = LastToken<bool?>(initLine);
                break;
            case "BO":
                relinquishDefault = SecondToken<double>(initLine);
                closedForStart = ThirdToken<bool?>(initLine);
                ignoreRelinquishDefault = !ForthToken<bool?>(initLine);//ignoreRelDef would be reverse of use initial value
                break;
            case "MSO":
                closedForStart = SecondToken<bool?>(initLine);
                relinquishDefault = ThirdToken<double>(initLine);
                ignoreRelinquishDefault = !ForthToken<bool?>(initLine);//ignoreRelDef would be reverse of use initial value
                break;
            case "AD":
                relinquishDefault = ThirdToken<double>(initLine);
                ignoreRelinquishDefault = false;
                break;
            case "BD":
                relinquishDefault = SecondToken<double>(initLine);
                adjustDisabled = LastToken<bool?>(initLine);
                ignoreRelinquishDefault = false;
                break;
            case "AOS":
                relinquishDefault = FirstToken<double>(initLine);
                ignoreRelinquishDefault = !ThirdToken<bool?>(initLine);//ignoreRelDef would be reverse of use initial value
                break;
        }
    }

Upvotes: 2

Views: 16452

Answers (4)

Ben Aaronson
Ben Aaronson

Reputation: 6975

I believe this question is asking about having to assign values inside the method with out parameters, and whether there is any way to circumvent that, i.e. by leaving the values unassigned rather than explicitly having to assign them null.

e.g. Do you have to write:

public bool OutMethod(out int? output)
{
    output = null;
    return true;
}

Or is there a way to do something like:

public bool OutMethod(out int? output)
{
    return true;
}

With the same result?

Short answer is no, this can't be avoided. See the documentation for an example which includes assigning null explicitly, and states:

Note that the third argument is assigned to null. This allows methods to return values optionally.

Examples of this can also be found in the .NET framework. e.g. Dictionary<TKey,TValue>'s TryGetValue method:

public bool TryGetValue(TKey key, out TValue value)
{
    int num = this.FindEntry(key);
    if (num >= 0)
    {
        value = this.entries[num].value;
        return true;
    }
    value = default(TValue);
    return false;
}

Note the explicit assignment to default(TValue)

Upvotes: 2

Edin
Edin

Reputation: 1496

The explanation is quite simple: You can do it. Just assign an out parameter inside the method to the desired value.

Now you can ask yourself, why can't we do that in the method signature? Well, let us have a look at the normal optional value parameters. They are assigned a predefined value if they are not assigned by the caller. So caller knows the value, which will be passed if the parameter is not set explicitly. It has a control of it and it is responsible for that decision.

Since the caller of the method is NOT responsible for the assignment of the out parameter, it makes no sense to offer a default value. The only thing you could achieve, by having a default out parameter value is to let the caller know what one of the possible values of that out parameter would be. But would that make sense? When will this value be used? Under what condition? All this is still hidden from the caller. So there is no real benefit from having an optional out parameter value, apart from having a possiblity to set it inside the signature instead of the method body.

So, having that said, the following would not make much sense:

public bool TrySomething(out string outObject = "default value") { ... }

However, what would be cool is to allow the following method

public bool TrySomething(out string outObject) { ... }

to be invoked as follows:

bool result = TrySomething();

And have that behind the scenes equivalent to:

string dummyWhichWillNeverBeUsed;
bool succeeded = TrySomething(out dummyWhichWillNeverBeUsed);

Unfortunately, that is not allowed.

And of course, as explained in other answers, you can always have overloads without out parameter.

Upvotes: 0

Igor
Igor

Reputation: 15893

Use the overload with signature that does not have out parameters:

private static void GetInitInfo(string initLine, string ncPointType)
{
  double? relinquishDefault;
  bool? ignoreRelinquishDefault;
  bool? closedForStart;
  bool? adjustDisabled;

  GetInitInfo( initLine, ncPointType, out relinquishDefault, out ignoreRelinquishDefault, 
    out closedForStart, out adjustDisabled);
}

Upvotes: 1

Xela
Xela

Reputation: 2382

This isn't not allowed according to the C# 4.0 Spec: section 21.1.

A workaround is to overload with another method that does not have out or ref parameters.

Upvotes: 2

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