How to assign a null to 'var' datatype ?

Question

3.00/5 (2 votes)

See more:

I am using VS2008.
I want to assign a null value to var datatype. How to do it? Here is my problem.
[Solution 2 explains why you cannot do it — SA]

C#

class cl
{
buildmenu()
{
if(condition=1)
var vv=some linq query                          -----A
else
if(condition=2)
var vv=some linq query                          -----B
}

...
...
...
foreach (var vx in vv)                           ----C
{

}}

Now if you see A and B, they are local and i cannot use it location C(since A and B were defined locally).
I also cant define var vv globally since it is not allowed.

What should i do ???? I want to assign a null value to var datatype so that whatever result comes out from A or B, could be used at C. [Solution 4 explains what to do — SA]

Quick help will be appreciated.

Regards.

Posted 3-Jan-12 21:13pm

shikhar gilhotra

Updated 5-Jan-12 5:12am

Sergey Alexandrovich Kryukov

v6

Add a Solution

Comments

Sergey Alexandrovich Kryukov 4-Jan-12 3:23am

Why told you var is a data type? It is not a type.
--SA

Amir Mahfoozi 4-Jan-12 4:12am

The foreach cannot be out of all functions so define a parameter for its function and pass vv to it. all linq results are implementing IEnumerable so you can define its parameter as IEnumerable and also add another parameter that says the type of vv items.

3 solutions

Solution 4

shikhar gilhotra wrote:
I also can't define var vv globally since it is not allowed. What should I do?

Apparently, in my answer I've explained what you cannot do and why, see Solution 2. I did not give you and idea on what to do. I'll fix this problem now.

Let's consider the very first Microsoft sample explaining the use of LINQ,
http://msdn.microsoft.com/en-us/library/bb397906.aspx[^].

Here is that sample, written using var:

C#

static void Main(string[] args) {
    int[] numbers = new int[7] { 0, 1, 2, 3, 4, 5, 6 };
    var numQuery =
        from num in numbers
        where (num % 2) == 0
        select num;
    foreach (int num in numQuery)
        Console.Write("{0,1} ", num);
} //Main

Now, you want to make an if branch with different queries. You correctly explain why you cannot do it with var. Let's be logical. It means that if you want to use the similar code logic and structure, you will need to exclude var by using explicit type. Here is how:

C#

static void Sample(bool condition) {
    int[] numbers = new int[7] { 0, 1, 2, 3, 4, 5, 6 };
    // here is the declaration in question:
    // you don't need to assign null to numQuery
    // because you don't use unassigned value of it anyway, but could be
    // IEnumerable<int> numQuery = null;
    IEnumerable<int> numQuery;
    if (condition)
        numQuery =
            from num in numbers
            where (num % 2) == 0
            select num;
    else
        numQuery =
            from num in numbers
            where (num > 4)
            select num;
    foreach (int num in numQuery)
        Console.Write("{0,1} ", num);
} //Sample

It is important to understand, that the run-time type is different from the compile-time type declared in the code sample shown above. You can use the nearest common base type which is assigned-compatible with the operand of your foreach container.

The problem is reduced to the problem of selection of appropriate query type for the explicit declaration. The rules are very well explained here: http://msdn.microsoft.com/en-us/library/bb397924.aspx[^].

Good luck,

—SA

Posted 5-Jan-12 4:52am

Sergey Alexandrovich Kryukov

Updated 5-May-12 15:32pm

v8

Solution 1

You can use Nullable types in .NET. Ref: http://msdn.microsoft.com/en-us/library/2cf62fcy%28v=vs.80%29.aspx[^]

Posted 3-Jan-12 21:14pm

Ganesan Senthilvel

Comments

shikhar gilhotra 4-Jan-12 3:17am

didnt get little help ;-(

Sergey Alexandrovich Kryukov 4-Jan-12 3:30am

...because this answer is completely irrelevant. Please ignore it to avoid confusion.
--SA

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Sergey Alexandrovich Kryukov · Accepted Answer · 2012-01-03T21:29:00

There is not such thing as var type. This is not a type at all. This is a keyword used for type inference, see http://en.wikipedia.org/wiki/Type_inference[^], http://msdn.microsoft.com/en-us/library/ms364047%28v=vs.80%29.aspx[^].

Using this keyword is strictly equivalent to explicit type name of the type which would be inferred by a compiler from the context if this is possible. As null is ambiguous (any nullable of reference type is assignment-compatible with null), inference is impossible, so var can not be used.

It is important to understand that using var does not mean going beyond strict typing. When a type is inferred, is treated like any other statically known type.

[EDIT] I've realized that you have a related problem to be solved anyway. You correctly explain it at the end of the text of the question. Please see Solution 4.

—SA