How to check for null

  • Thread starter Thread starter tshad
  • Start date Start date
T

tshad

I am sending data to my Sql Server and in some cases get null back.

In this case, I am sending back an int, but if it is null, I get a
"Specified cast not valid".

So I did the following:
if (parameters[7].Value == null)
return -5;
else
return (int)parameters[7].Value;

but get the same error.

How do I test for null here?

Thanks,

Tom
 
tshad said:
I am sending data to my Sql Server and in some cases get null back.

In this case, I am sending back an int, but if it is null, I get a
"Specified cast not valid".

So I did the following:
if (parameters[7].Value == null)
return -5;
else
return (int)parameters[7].Value;
Click to expand...

I also tried:

return (parameters[7].Value ?? 0);

and

return ((int)parameters[7].Value ? (int)parameters[7].Value : 0);

The error here was:

User.cs(198,13): error CS0029: Cannot implicitly convert type 'int' to
'bool'

Tom
 
What's happening in your code, roughly speaking, is this:

ADO.NET represents each cell of data coming back from a SELECT as
either an object of the correct data type for the column (in your case,
a boxed int) or as DBNull.Value. DBNull is a singleton class, so there
only ever is one instance of DBNull.Value for your whole program.

So, when you try to grab data from a cell (row[column]) of an ADO.NET
table, you first have to check to see what type it is. Since there are
only two possibilities, you can test:

int param7;
if (parameters[7] == DBNull.Value)
{
// ... handle null case ...
param7 = -1;
}
else
{
param7 = (int)(parameters[7]);
}

You can't cast the cell to (int) until you determine that it's _not_
DBNull.Value. If it's not DBNull.Value, the only other possibility is
that it is a value of the correct type for the column, so it's safe to
cast it to int.

By the way, you can do an == comparison against DBNull.Value because,
even though it's a reference type, it's a singleton, so the ==
reference comparison will get the job done. You don't need to call
DBNull.Value.Equals(), although that will do the same thing.

In C# 2.0 all of this is made much simpler by nullable types. You can
declare an int? instead of an int and do this:

int? param7 = (int?)(parameters[7]);

which I'm glad for, since I'm sick and tired of testing for
DBNull.Value. (Caveat: I'm not using C# 2.0 yet. Apologies for any
errors in syntax in the above line.)

In C# 1.1 I've gotten around this problem by writing static utility
methods like this one:

public static int DBValueToInt(object cellValue, int defaultValue)
{
if (cellValue == DBNull.Value)
{
return defaultValue;
}
else
{
return (int)cellValue;
}
}

So now I just write:

int param7 = DBValueToInt(parameters[7], -1);

to get the same effect as the first bit of code, above.
 
tshad said:
I also tried:

return (parameters[7].Value ?? 0);

and

return ((int)parameters[7].Value ? (int)parameters[7].Value : 0);

The error here was:

User.cs(198,13): error CS0029: Cannot implicitly convert type 'int' to
'bool'
Click to expand...

In c# 0's and nulls are not automatically converted to boolean
false, as in c or c++. The ternary operator above expects boolean,
while you give it an integer. You should check if (int)Value == 0,
or value == null, or something like that.

HTH, Tom
 
if (parameters[7] == DBNull.Value)
{
Click to expand...
or:
if (parameters[7] is DBNull)

also you have to use DBNull.Value if you want to pass a NULL to a
SQL-Satement in a parameter:
parameter[7] = DBNull.Value;
 
Bruce Wood said:
What's happening in your code, roughly speaking, is this:

ADO.NET represents each cell of data coming back from a SELECT as
either an object of the correct data type for the column (in your case,
a boxed int) or as DBNull.Value. DBNull is a singleton class, so there
only ever is one instance of DBNull.Value for your whole program.

So, when you try to grab data from a cell (row[column]) of an ADO.NET
table, you first have to check to see what type it is. Since there are
only two possibilities, you can test:

int param7;
if (parameters[7] == DBNull.Value)
{
// ... handle null case ...
param7 = -1;
}
else
{
param7 = (int)(parameters[7]);
}
Click to expand...

That's actually how I do it in VB.Net. Not sure why I didn't think to do it
here.
You can't cast the cell to (int) until you determine that it's _not_
DBNull.Value. If it's not DBNull.Value, the only other possibility is
that it is a value of the correct type for the column, so it's safe to
cast it to int.

By the way, you can do an == comparison against DBNull.Value because,
even though it's a reference type, it's a singleton, so the ==
reference comparison will get the job done. You don't need to call
DBNull.Value.Equals(), although that will do the same thing.

In C# 2.0 all of this is made much simpler by nullable types. You can
declare an int? instead of an int and do this:

int? param7 = (int?)(parameters[7]);

which I'm glad for, since I'm sick and tired of testing for
DBNull.Value. (Caveat: I'm not using C# 2.0 yet. Apologies for any
errors in syntax in the above line.)
Click to expand...

Me too (in VB.Net as well).

Do you know if 2.0 will have this in VB.Net also?
In C# 1.1 I've gotten around this problem by writing static utility
methods like this one:

public static int DBValueToInt(object cellValue, int defaultValue)
{
if (cellValue == DBNull.Value)
{
return defaultValue;
}
else
{
return (int)cellValue;
}
}
Click to expand...

Do you have to put this in all your programs or do you have it set up as
part of a class?

Thanks,

Tom
So now I just write:

int param7 = DBValueToInt(parameters[7], -1);

to get the same effect as the first bit of code, above.
Click to expand...
 
public static int DBValueToInt(object cellValue, int defaultValue)
Do you have to put this in all your programs or do you have it set up as
part of a class?
Click to expand...

You can create a

public sealed class DataUtilities
{
private DataUtilities() { }

... put your DBValueTo... methods here...
}

class to hold useful static methods that don't really belong anywhere
else. Then you just say

int param7 = DataUtilities.DBValueToInt(parameters[7], -1);
 
Bruce Wood said:
Do you have to put this in all your programs or do you have it set up as
part of a class?
Click to expand...

You can create a

public sealed class DataUtilities
{
private DataUtilities() { }

... put your DBValueTo... methods here...
}

class to hold useful static methods that don't really belong anywhere
else. Then you just say

int param7 = DataUtilities.DBValueToInt(parameters[7], -1);
Click to expand...

Is that similar to a share?

I have some General purpose routines that are not object, but are in my
MyFunctions Namespace

*************************************************************************************
NameSpace MyFunctions

Public Class BitHandling

'*----------------------------------------------------------*
'* Name : BitSet *
'*----------------------------------------------------------*
'* Purpose : Sets a given Bit in Number *
'*----------------------------------------------------------*
Public Shared Function BitSet(Number As Integer, _
ByVal Bit As Integer) As Long
If Bit = 31 Then
Number = &H80000000 Or Number
Else
Number = (2 ^ Bit) Or Number
End If
BitSet = Number
End Function

**************************************************************

Is that the same thing?

Thanks,

Tom
 
tshad said:
tshad said:
I am sending data to my Sql Server and in some cases get null back.

In this case, I am sending back an int, but if it is null, I get a
"Specified cast not valid".

So I did the following:
if (parameters[7].Value == null)
return -5;
else
return (int)parameters[7].Value;
Click to expand...

I also tried:

return (parameters[7].Value ?? 0);

and

return ((int)parameters[7].Value ? (int)parameters[7].Value : 0);

The error here was:

User.cs(198,13): error CS0029: Cannot implicitly convert type 'int' to
'bool'

Tom
Click to expand...

I just ended up always testing for null - which is probably best.

************************************************************************************
public class User
{
private string clientID = "";
private int userID = 0;
private long companyID= 0;
private string firstName = "";
private string lastName = "";
private string userName = "";
private string secretQuestion = "";
private string secretAnswer = "";
private string email = "";
private string password = "";
private bool firstTime = true;
private bool emailHtml = true;
private string dateCreated;
private string connectionString = "";
private int status = 0;

....

private void LoadDetails (DataRow user)
{
if (user["FirstName"] != DBNull.Value)
firstName = (string)user["FirstName"];
if (user["LastName"] != DBNull.Value)
lastName = (string)user["LastName"];
if (user["UserName"] != DBNull.Value)
userName = (string)user["UserName"];
if (user["SecretQuestion"] != DBNull.Value)
secretQuestion = (string)user["SecretQuestion"];
if (user["SecretAnswer"] != DBNull.Value)
secretAnswer = (string)user["SecretAnswer"];
if (user["Email"] != DBNull.Value)
email = (string)user["Email"];
if (user["Password"] != DBNull.Value)
password = (string)user["Password"];
if (user["DateCreated"] != DBNull.Value)
dateCreated = user["DateCreated"].ToString();
if (user["UserID"] != DBNull.Value)
userID = (int)user["UserID"];
if (user["FirstTime"] != DBNull.Value)
firstTime = (bool)user["FirstTime"];
if (user["emailHtml"] != DBNull.Value)
emailHtml = (bool)user["emailHtml"];
if (user["CompanyID"] != DBNull.Value)
companyID = (int)user["CompanyID"];
if (user["ClientID"] != DBNull.Value)
clientID = (string)user["ClientID"];
}
******************************************************************

Seems like the best way to handle it to make sure there are no errors.

Thanks,

Tom
 
Have you guys tried this?

private void LoadDetails (DataRow user)
{
if (dr.IsNull("emailHtml"))
{
//it's null;
}
else
{
//it's not null;
}
}


tshad said:
tshad said:
tshad said:
I am sending data to my Sql Server and in some cases get null back.

In this case, I am sending back an int, but if it is null, I get a
"Specified cast not valid".

So I did the following:
if (parameters[7].Value == null)
return -5;
else
return (int)parameters[7].Value;
Click to expand...

I also tried:

return (parameters[7].Value ?? 0);

and

return ((int)parameters[7].Value ? (int)parameters[7].Value : 0);

The error here was:

User.cs(198,13): error CS0029: Cannot implicitly convert type 'int' to
'bool'

Tom
Click to expand...

I just ended up always testing for null - which is probably best.

************************************************************************************
public class User
{
private string clientID = "";
private int userID = 0;
private long companyID= 0;
private string firstName = "";
private string lastName = "";
private string userName = "";
private string secretQuestion = "";
private string secretAnswer = "";
private string email = "";
private string password = "";
private bool firstTime = true;
private bool emailHtml = true;
private string dateCreated;
private string connectionString = "";
private int status = 0;

...

private void LoadDetails (DataRow user)
{
if (user["FirstName"] != DBNull.Value)
firstName = (string)user["FirstName"];
if (user["LastName"] != DBNull.Value)
lastName = (string)user["LastName"];
if (user["UserName"] != DBNull.Value)
userName = (string)user["UserName"];
if (user["SecretQuestion"] != DBNull.Value)
secretQuestion = (string)user["SecretQuestion"];
if (user["SecretAnswer"] != DBNull.Value)
secretAnswer = (string)user["SecretAnswer"];
if (user["Email"] != DBNull.Value)
email = (string)user["Email"];
if (user["Password"] != DBNull.Value)
password = (string)user["Password"];
if (user["DateCreated"] != DBNull.Value)
dateCreated = user["DateCreated"].ToString();
if (user["UserID"] != DBNull.Value)
userID = (int)user["UserID"];
if (user["FirstTime"] != DBNull.Value)
firstTime = (bool)user["FirstTime"];
if (user["emailHtml"] != DBNull.Value)
emailHtml = (bool)user["emailHtml"];
if (user["CompanyID"] != DBNull.Value)
companyID = (int)user["CompanyID"];
if (user["ClientID"] != DBNull.Value)
clientID = (string)user["ClientID"];
}
******************************************************************

Seems like the best way to handle it to make sure there are no errors.

Thanks,

Tom
Click to expand...
 

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