Checking against null in generic code

  • Thread starter Thread starter Adam Badura
  • Start date Start date
A

Adam Badura

How to check if a value of generic type is null f it is not known if it
is a reference type or simple type.
Lets look at the example:

class Test<Type1, Type2> where Type1 : IEnumerable<Type2>
{
public Fun(Type1 item)
{
// a check is here needed
foreach (Type2 part in item)
{
/*...*/
}
}
}

If then I use code like:

new Test<string, char>().Fun(null);

I obviously get an exception because item used in foreach is null and cannot
be null. However if I add an if before foreach to check against null I
receive a warning (saddly I am not sure and cannot check now if the warning
is issued by Visual Studio, Code Analysys in Team Suite or Resharper) that
the code is not written very well because Type1 does not have to be a class
type.
The propsed solution o use not
if (item == null)
but rather
if (item == default(Type1))
(or something like this - I am not sure if the syntax was exactly like this)
does not work. It is good if Type1 is class type, but if it is struct type
then the check might yield true for a default value which might be
enumerable.

What I wanted was to check and for null throw ArgumentNullException. I
can achive this by catching null from foreach and rethrowing but I think it
is not an elegant solution. So I am asking. How to add that check against
null?

Adam Badura
 
Hello,

you can use

EqualityComparer<T>.Default.Equals(item, default(T))

Kind regards,
Henning Krause
 
EqualityComparer<T>.Default.Equals(item, default(T))

Isn't this the same as writing
item == default(T)

And even if not I don't get it. This will not resolve the problem of
structs. If T is struct type then it cannot be null and always has a value.
So it always can be used in foreach.
Basing the check condition on this condition would cause the "if" to be
true for default valued structs and in result cause to throw
ArgumentNullExcpetion. This is bad in 2 ways: struct value cannot be null,
so this exception seems to be inappropriate and the valu could be used in
foreach so no throwing is needed.

Adam Badura
 
Hello Adam,

Adam Badura said:
Isn't this the same as writing
item == default(T)
Click to expand...

No. Because your line of code will fail with "Error 1 Operator '==' cannot
be applied to operands of type 'Type1' and 'Type1'"
And even if not I don't get it. This will not resolve the problem of
structs. If T is struct type then it cannot be null and always has a
value. So it always can be used in foreach.
Click to expand...

Yes, you are right with that.

What do you think of this?

class Test<Type1, Type2> where Type1 : IEnumerable<Type2>
{
public void Fun(Type1 item)
{
IEnumerable<Type2> enumerable = item;

if (enumerable == null) throw new
ArgumentNullException("item");

// a check is here needed
foreach (Type2 e in enumerable)
{
/*...*/
}
}
}
If it's a value type, it will be boxed. No harm done. If Type1 is a class,
it will be checked for null.

Kind regards,
Henning Krause
 
By the way. I am new to this group. What do does [MV.... in author
mean?
Click to expand...

MVP = "Most Valuable Professional" = Microsoft awarded title - Typically
given for community work in a given area.
 
No. Because your line of code will fail with "Error 1 Operator '==' cannot
be applied to operands of type 'Type1' and 'Type1'"
Click to expand...

Yes. You are right.
What do you think of this?

class Test<Type1, Type2> where Type1 : IEnumerable<Type2>
{
public void Fun(Type1 item)
{
IEnumerable<Type2> enumerable = item;

if (enumerable == null) throw new
ArgumentNullException("item");

// a check is here needed
foreach (Type2 e in enumerable)
{
/*...*/
}
}
}
If it's a value type, it will be boxed. No harm done. If Type1 is a class,
it will be checked for null.
Click to expand...

That is what I needed. Prety simple. Shame I didn't thought on that
first before asking. :)

By the way. I am new to this group. What do does [MV.... in author mean?

Adam Badura
 
Back
Top