Can I cast from DBNull to a Nullable Bool in one line?

Question

I have a database query which will either return NULL or a boolean (bit) value.

I wish to store this value in a variable of type Nullable<bool> in C#.

I can't seem to find an acceptable mix of explict casts and conversions that do this in a simple way without Exceptions being thrown.

Can it be done in one readable line?

EDIT: Code as requested

private Nullable<bool> IsRestricted;
...//data access
IsRestricted = (bool?)DataBinder.GetPropertyValue(dataObj, "IsRestricted");

or perhaps

IsRestricted = (bool?)(bool)DataBinder.GetPropertyValue(dataObj, "IsRestricted");

If it's just a matter of coding style, write a `ToNullable` method. Use [this](http://geekswithblogs.net/WillSmith/archive/2008/03/04/generic-conversion-for-dbnull-and-nullable-types.aspx) as a starting point. — Robert Harvey, Jan 30 '13 at 17:23
Will the db query code really return `NULL`, or does it return `DBNull.Value`? — Bridge, Jan 30 '13 at 17:25
Funny enough that Microsoft [describes](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/nullable-value-types) `Nullable` as a use-case for `NULL` DB column values. — argonym, Jul 24 '20 at 10:24

giammin · Accepted Answer · 2013-01-30T17:57:50.317

13

assuming you have a datareader dr:

bool? tmp = Convert.IsDBNull(dr["dbnullValue"]) ? null: (bool?) dr["dbnullValue"];

---ADDED----

or maybe you can use the ?? if you don't have to check for DBNull but i'm not sure compiler will like this (i cannot test it now)

bool? tmp = dr["dbnullValue"] ?? (bool?) dr["dbnullValue"];

edited Jan 30 '13 at 17:57

answered Jan 30 '13 at 17:31

giammin

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This looks to be what I want. Will test it. – Widor Jan 30 '13 at 17:42
Your second snippet won't compile. `??` doesn't know anything about `DBNull`. – svick Jan 30 '13 at 19:35
1

Your first example does what I need - the second is one I tried without success. – Widor Jan 31 '13 at 11:59
1

the second snippet is in case you don't have to check for dbnull but only for null! – giammin Jan 31 '13 at 16:10
The first alternative is loooong, the second one doesn't seem to work. -1 I think answer of SLaks is better. – Al Kepp Feb 21 '19 at 09:26
@giammin I think unsafe is to repeat twice the string with column name dr["column"]. If you make a typo, you have a problem. – Al Kepp Feb 22 '19 at 16:06

score 6 · Answer 2 · edited May 23 '17 at 11:45

6

You could write value as bool?.
This will return null if value is not of type bool.

Note that this is somewhat inefficient.

edited May 23 '17 at 11:45

Community

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answered Jan 30 '13 at 17:21

SLaks

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I don't like this, because it's unsafe. If something changes and the result is some other type, I think exception is much better than `null`. – svick Jan 30 '13 at 17:24
2

@svick: Then you need more complexity: `value is DBNull ? null : (bool?)value` – SLaks Jan 30 '13 at 17:28

score 0 · Answer 3 · answered Jan 30 '13 at 19:59

0

 while (reader.Read()) {
    bool? IsRestricted = (reader.IsDBNull(reader.GetOrdinal("IsRestricted"))) ? (null) : ((bool)reader.GetOrdinal("IsRestricted")));
 }

answered Jan 30 '13 at 19:59

Lajos Arpad

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score 0 · Answer 4 · answered Jan 30 '13 at 20:30

I use extension methods for this issue.

var isRestricted = dataRecord.GetNullableValue<bool>("IsRestricted");

There is code of GetNullableValue method:

    public static Nullable<TValue> GetNullableValue<TValue>(
        this IDataRecord record, 
        string name) where TValue : struct
    {
        return record.GetValue<TValue, Nullable<TValue>>(name);
    }

And there is also a simple code for GetValue method:

        private static TResult GetValue<TValue, TResult>(
        this IDataRecord record,
        string name)
    {
        var result = record[name];
        return !result.Equals(DBNull.Value) ? (TResult)result : default(TResult);
    }

Why `TValue` parameter on the `GetValue` method? Also, why `Nullable` rather than `T?` ? — kͩeͣmͮpͥ ͩ, May 14 '14 at 09:16

score 0 · Answer 5 · answered Dec 18 '19 at 12:58

0

You can just do the following

bool? myNullableBoolean = SqlConvert.ToType<bool?>(reader["myNullableBooleanColumn"]);

answered Dec 18 '19 at 12:58

JB's

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Can I cast from DBNull to a Nullable Bool in one line?

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