Nullable strings as parameters

  • Thread starter Thread starter Mark Rae
  • Start date Start date
M

Mark Rae

Hi,

In v1.1 I used to simulate nullable datatypes with structs. e.g.

public struct NullableString
{
private readonly String pstrValue;
private readonly bool blnHasValue;
public NullableString(String strValue)
{
this.pstrValue = strValue;
this.blnHasValue = true;
}
public String Value {get{return this.pstrValue;}}
public bool HasValue {get{return blnHasValue;}}
};

However, now that we have these natively in v2, I'm trying to use them as
part of my database abstraction layer. I have a class which begins:

public class CSale
{
private String? pstrTransactionID;
public String? strTransactionID {get{return
pstrTransactionID;}set{pstrTransactionID = value;}}

private DateTime? pdtmPayment;
public DateTime? dtmPayment{get{return pdtmPayment;}set{pdtmPayment =
value;}}

private int? pintItems;
public int? intItems {get{return pintItems;}set{pintItems = value;}}

private decimal? pcurFee;
public decimal? curFee{get{return pcurFee;}set{pcurFee = value;}}

// etc
}

However, this doesn't compile. Instead, the compiler generates the following
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>'

IntelliSense still works, though, i.e. if I type a period after
strTransactionID, it drops down the list of properties and methods e.g.
HasValue, Value etc.

If I comment out the two lines relating to the String parameter, the class
compiles OK.

Any assistance gratefully received.

Mark
 
However, this doesn't compile. Instead, the compiler generates the following
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>'
Click to expand...

Only value types (i.e. structs) can be made nullable (as these cannot
otherwise be set to 'null'); it is rather meaningless to use string? as
string is a reference type anyway, so you can just set a string object
to null...so, just don't make your string members nullable explicitly,
and you'll still be able to assign them to null as you require.
 
Only value types (i.e. structs) can be made nullable (as these cannot
otherwise be set to 'null'); it is rather meaningless to use string? as
string is a reference type anyway, so you can just set a string object
to null...so, just don't make your string members nullable explicitly,
and you'll still be able to assign them to null as you require.
Click to expand...

I see.

For the other various datatypes, I use variations of the following code to
build up a SqlParameters collection:

mobjSqlParameter = new SqlParameter("@pdtmSale", SqlDbType.SmallDateTime);
mobjSqlParameter.IsNullable = true;
mobjSqlParameter.Value = dtmSale.HasValue ? dtmSale.Value :
(object)DBNull.Value;
maobjSqlParameters.Add(mobjSqlParameter);

Am I able to do something similar with strings?
 
For the other various datatypes, I use variations of the following code to
build up a SqlParameters collection:

mobjSqlParameter = new SqlParameter("@pdtmSale", SqlDbType.SmallDateTime);
mobjSqlParameter.IsNullable = true;
mobjSqlParameter.Value = dtmSale.HasValue ? dtmSale.Value :
(object)DBNull.Value;
maobjSqlParameters.Add(mobjSqlParameter);

Am I able to do something similar with strings?
Click to expand...

Yes; for the third line you should just be able to use:
mobjSqlParameter.Value = myStr == null ? (object)DBNull.Value :
myStr;
 
Yes; for the third line you should just be able to use:
mobjSqlParameter.Value = myStr == null ? (object)DBNull.Value :
myStr;
Click to expand...

Cool! Thanks a lot for your assistance.
 
Back
Top