Data source controls and parameter type conversion

Question

Hello,

Q1 - In the code example below runtime converts a value between two incompatible types:

  <SelectParameters>
 <asp:Parameter Name="City" Type="Int32" />
 </SelectParameters>



protected void SqlDataSource2_Selecting(object sender,
       SqlDataSourceSelectingEventArgs e)
{e.Command.Parameters["@City"].Value = "100";}

Exception I got:

“Conversion failed when converting the nvarchar value 'Seattle' to data type int.”

A) The above exception suggests that runtime did manage to convert a value of type String into a value of type Int32, and thus the exception happened on SqlServer?!

B) Since String and Int32 are incompatible types, why did runtime perform conversion from String to Int32 in the first place?

Doesn’t the fact that we’re dealing with incompatible type make runtime “realize” that app most likely has a bug, similary to the way compiler “realizes” that it’s dealing ( in the code below ) with two incompatible types:

String s = ”something”;
int i = (int)s; //error

Q2:

A) public void GetEmployee( int EmployeeID );

<asp:ObjectDataSource  SelectMethod=”GetEmployee” …>
  <SelectParameters>
    <asp:ControlParameter Name = ”EmployeeID” ...>
  </SelectParameters>

If for whatever reason EmployeeID parameter is NULL, ObjectDataSource will convert Null to zero and passed it as argument to GetEmployee() method.

Why does runtime make such a conversion? Wouldn't throwing an exception made more sense?

B) “Use the ConvertEmptyStringToNull property to specify whether an empty string value is automatically converted to null when the data field is updated in the data source.”
I don’t quite understand the usefulness of this property. Why would empty string indicate that we want null to be inserted into source’s data field? I’d assume that this data field is of type String? Then why not also have ConvertZeroInt32ToNull etc?

Q3:

<asp:SqlDataSource ID="sourceEmployees" runat="server"
     ProviderName="System.Data.SqlClient"
     ConnectionString="<%$ ConnectionStrings:Northwind %>"
     SelectCommand="SELECT EmployeeID, FirstName,LastName,
     Title, City FROM Employees WHERE City=@City">
         <SelectParameters>
             <asp:Parameter Name="City"/>
         </SelectParameters>
</asp:SqlDataSource>

A) I assume that when you don’t specify of which type Parameter instance “City” is, it is automatically of type Object, which means it can later be assigned value of any type. Thus if “City” is later ( say inside SqlDataSource2_Selecting() event handler ) assigned a value of a wrong type, this wrong assignment will only be detected on Sql server, and not before ( of course Sql server will report that error back to web server )?

B) If we create a SqlParameter instance of type NVarChar(20) and want to pass this parameter to a stored procedure, will Ado.net pass to a stored procedure just the value of this parameter, or will it also somehow inform the procedure the exact type of this parameter ( which is NVarChar(20))?

thanx

Answer 1

I'll just try to answer your first question -- “Conversion failed when converting the nvarchar value 'Seattle' to data type int.”

Since I don't see "Seattle" mentioned anywhere, I'd guess that this error is actually coming from the stored procedure you're calling -- ie. whatever you're using @City for in the proc is the problem. It sounds like instead of doing something like "where CityID=@City", you're doing a "where CityName=@City", and it's complaining that it can't convert one of the values of CityName ("Seattle") to an integer to compare it with @City.

Answer 2

I'd guess that this error is actually coming from the stored procedure you're calling -- ie. whatever you're using @City for in the proc is the problem. It sounds like instead of doing something like "where CityID=@City", you're doing a "where CityName=@City", and it's complaining that it can't convert one of the values of CityName ("Seattle") to an integer to compare it with @City.

I used incompatible types on purpose just to get my point across --> which is ‘why does runtime even bother to try to convert a value from one incompatible type to another, since the fact that user assigned string to variable of type integer, suggests that it is most likely a bug and not something user did knowingly'?

Answer 3

This is a SQL Server error. You've hit a datatype precedence issue.

The clue is here:

SELECT EmployeeID, FirstName,LastName, Title, City FROM Employees WHERE
City=@City --here exactly

The City column is nvarchar The parameter is explicitly set to int

Adding it all together, you get:

SELECT EmployeeID, FirstName,LastName, Title, City FROM Employees WHERE
City=100

By the rules of datatype precedemce, the database engine tries to change all the values for City to "int". And fails, of course.

For your points though:

• Q1: Same issue. Nowhere have you told SQL Server that the "100" should be string
• Q2: Empty string casts to zero in SQL. Send dbnull.value if you want a SQL null
• Q3a. SQL has no object data type. It looks at 100 and decides "int"
• Q3b. Yes, use a stored proc and define it correctly.

Overall, SQL Server is behaving exactly as advertised. In this case, you simply are not giving enough information.

Edit: Stored proc usage

You tell SQL what the data type is when you add the parameter to the parameters collection. You define (and create) the stored proc to have the nvarchar parameter.