Linq compare nullable string to string

Question

I've got the following LINQ:

        var items = from n in db.Roles
                    where n.importID == importID
                    select n;

The "importID" is a string as parameter and a string? for the database object. If importID is null in Database there shall be no match. How to achieve this?

Compiler message:

Operator '==' cannot be applied to operands of type 'string?' and 'string'

It is... at least using Entity Framework this means a string that is nullable (e.g. can be set to DbNull). — SDwarfs
– SDwarfs, Commented Sep 18, 2012 at 8:27

RB. · Accepted Answer · 2012-09-18 08:28:54Z

6

A string is a reference type, and so is already nullable (i.e. you can set it to null). The only purpose of the Nullable type is to allow value types (such as integers, doubles, etc.) to be set to null. The fix is to declare importId as string, not string?.

edited Sep 18, 2012 at 8:28

answered Sep 18, 2012 at 8:23

RB.

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5 Comments

SDwarfs Over a year ago

I'm aware of that. And yes, i want to have importID to be nullable (as not all entries will be imported ones [user created] and therefore have an importID).

Adam Over a year ago

@StefanK. but it is impossible to define a nullable string when string is already nullable. The compiler doesn't allow it!

SDwarfs Over a year ago

The compiler does... there isn't even a warning.

Adam Over a year ago

string? and a value that can be DbNull are two very different things. See this question. string? will always give you a compiler error (

The type 'string' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable<T>'

).

SDwarfs Over a year ago

Hm, ok... as this is possibly a bug in Entity Framework, I'll test this and investigate it in more detail. Thanks at least for the warning...

JConstantine · Accepted Answer · 2012-09-18 08:21:44Z

4

var items = from n in db.Roles
            where n.importID.HasValue && n.importId.Value == importID
            select n;

answered Sep 18, 2012 at 8:21

JConstantine

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2 Comments

Adam Over a year ago

@StefanK. no, it's not. See the other answer.

SDwarfs Over a year ago

@codesparkle: I would consider your comment, if you would explain, why the code is compiling and seems logical.

Mayank Pathak · Accepted Answer · 2012-09-18 09:09:01Z

0

Also you can use .GetValueOrDefault()

In Short Query can be...

var items = from n in db.Roles
                where n.importID == importID.GetValueOrDefault()
                select n;

answered Sep 18, 2012 at 9:09

Mayank Pathak

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Comments

Pushpendra · Accepted Answer · 2012-09-18 09:24:45Z

0

var items = from n in db.Roles
                    where !string.IsNullOrEmpty(n.importID) && n.importID == importID
                    select n;

answered Sep 18, 2012 at 9:24

Pushpendra

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