When should an `out` parameter be used instead of returning a complex type?

Question

When should we use an out parameter in C#?

For example

bool TryGetValue(out object value);

vs.

class ReturnType
{
      public bool Found {get;set;}
      public object Value {get;set;}
}

ReturnType TryGetValue();

Apart from reducing the number of lines of code, when should an out parameter to be used and when it should be returned as return type?

Answer 1

Out can also be used for operations that might fail (typically found in methods starting Try*).

E.g. TryParse will return a bool indicating success/failure while using the out value as the result. This avoids having to throw exceptions.

Answer 2

If there's no general reason to encapsulate the multiple values together, other than for this call, then creating a separate type is probably over the top.

On the other hand, if you have multiple out parameters and a return value which all logically belong together (e.g. first name, last name, telephone number) then it would probably make sense to create an appropriate type (e.g. "Contact") and return that.

One alternative for the TryGetValue option is to use nullable value types, if the value in question is a value type. For instance, int.TryParse could have had a signature of:

int? TryParse(string text)

(with an overload taking an IFormatProvider of course).