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c#methodsparametersdefault-value
user667429
user667429

Reputation: 3113

How can I set default parameter value for an action

How can I set default parameter value for an action when that parameter is dictionary type?

for example:

public void Test(Dictionary<int, bool> dic) {
    ...
}

Upvotes: 4

Views: 6882

Answers (5)

weston
weston

Reputation: 54781

You could use null as special case, like in other answers, but if you still want to be able to call Test(null) and have different behaviour to calling Test(), then you must chain overload:

public void Test(Dictionary<int, bool> dic) {
  //optional, stops people calling Test(null) where you want them to call Test():
  if(dic == null) throw new ArgumentNullException("dic");
...
}

public void Test() {
   var defaultDic = new Dictionary<int, bool>();
   Test(defaultDic);
}

Upvotes: 4

Adam Houldsworth
Adam Houldsworth

Reputation: 64467

Making the assumption that you want to provide a non-null default in the method signature, you cannot do that with this type. However, you have two alternative solutions that immediately spring to mind.

1, use optional parameters with a default, this will have to be null for a Dictionary (and all other reference types except string I believe) and you will need logic inside the method to handle it:

public void Test(Dictionary<int, bool> dictionary = null)
{
    // Provide a default if null.
    if (dictionary == null)
        dictionary = new Dictionary<int, bool>();
}

Or, and the way I'd do it, just use the "old fashioned" method overloading. This allows you to differentiate between people not providing the argument and people providing a null argument:

public void Test()
{ 
    // Provide your default value here.
    Test(new Dictionary<int, bool>();
}

public void Test(Dictionary<int, bool> dictionary)
{

}

Optional parameters compile into overloaded methods at any rate, so they are almost semantically the same, it's just a preference as to where you wish to express your default value.

Upvotes: 0

Ahmed KRAIEM
Ahmed KRAIEM

Reputation: 10427

You can only use null as a default parameter value for reference types.

A default value must be one of the following types of expressions:

  • a constant expression;

  • an expression of the form new ValType(), where ValType is a value type, such as an enum or a struct;

  • an expression of the form default(ValType), where ValType is a value type.

MSDN

Upvotes: 1

Jason Evans
Jason Evans

Reputation: 29186

You cannot set the dictionary to anything other then NULL. If you do try, for example:

public void Test(Dictionary<int, bool> dic = new Dictionary<string, string> { { "1", "true" }}) or whatever, then you will see this error:

Default parameter value for 'dic' must be a compile-time constant.

So in that case, NULL is your only choice. However, doing this is pointless

public void Test(Dictionary<int, bool> dic = null)

At worst the incoming dic will be NULL anyway, if the caller hasn't instantiated a new instance, so there is no advantage to adding the NULL default anyway.

Upvotes: 0

roqz
roqz

Reputation: 1018

You can't give it a default value the way you want to because it has to be a compile time constant value, but you can do something like this:

private static bool Test(Dictionary<string, string> par = null)
    {
        if(par == null) par = GetMyDefaultValue();
        // Custom logic here
        return false;
    }

Upvotes: 5

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