[c#] Unable to cast object of type 'System.DBNull' to type 'System.String`

I got the above error in my app. Here is the original code

public string GetCustomerNumber(Guid id)
{
     string accountNumber = 
          (string)DBSqlHelperFactory.ExecuteScalar(connectionStringSplendidmyApp, 
                          CommandType.StoredProcedure, 
                          "GetCustomerNumber", 
                          new SqlParameter("@id", id));
     return accountNumber.ToString();
 }

I replaced with

public string GetCustomerNumber(Guid id)
{
   object accountNumber =  
          (object)DBSqlHelperFactory.ExecuteScalar(connectionStringSplendidCRM, 
                                CommandType.StoredProcedure, 
                                "spx_GetCustomerNumber", 
                                new SqlParameter("@id", id));
    if (accountNumber is System.DBNull)
    {
       return string.Empty;
    }
    else
    {
       return accountNumber.ToString();
    }
}

Is there a better way around this?

There is another way to workaround this issue. How about modify your store procedure? by using ISNULL(your field, "") sql function , you can return empty string if the return value is null.

Then you have your clean code as original version.

I suppose you can do it like this:

string accountNumber = DBSqlHelperFactory.ExecuteScalar(...) as string;

If accountNumber is null it means it was DBNull not string :)

Convert it Like

string s = System.DBNull.value.ToString();

You can use C#'s null coalescing operator

return accountNumber ?? string.Empty;

I use an extension to eliminate this problem for me, which may or may not be what you are after.

It goes like this:

public static class Extensions
{

    public String TrimString(this object item)
    {
        return String.Format("{0}", item).Trim();
    }

}

Note:

This extension does not return null values! If the item is null or DBNull.Value, it will return an empty String.

Usage:

public string GetCustomerNumber(Guid id)
{
    var obj = 
        DBSqlHelperFactory.ExecuteScalar(
            connectionStringSplendidmyApp, 
            CommandType.StoredProcedure, 
            "GetCustomerNumber", 
            new SqlParameter("@id", id)
        );
    return obj.TrimString();
}

String.Concat transforms DBNull and null values to an empty string.

public string GetCustomerNumber(Guid id)
{
   object accountNumber =  
          (object)DBSqlHelperFactory.ExecuteScalar(connectionStringSplendidCRM, 
                                CommandType.StoredProcedure, 
                                "spx_GetCustomerNumber", 
                                new SqlParameter("@id", id));

    return String.Concat(accountNumber);

 }

However, I think you lose something on code understandability

ExecuteScalar will return

• null if there is no result set
• otherwise the first column of the first row of the resultset, which may be DBNull.

If you know that the first column of the resultset is a string, then to cover all bases you need to check for both null and DBNull. Something like:

object accountNumber = ...ExecuteScalar(...);
return (accountNumber == null) ? String.Empty : accountNumber.ToString();

The above code relies on the fact that DBNull.ToString returns an empty string.

If accountNumber was another type (say integer), then you'd need to be more explicit:

object accountNumber = ...ExecuteScalar(...);
return (accountNumber == null || Convert.IsDBNull(accountNumber) ?     
         (int) accountNumber : 0;

If you know for sure that your resultset will always have at least one row (e.g. SELECT COUNT(*)...), then you can skip the check for null.

In your case the error message "Unable to cast object of type ‘System.DBNull’ to type ‘System.String`" indicates that the first column of your result set is a DBNUll value. This is from the cast to string on the first line:

string accountNumber = (string) ... ExecuteScalar(...);

Marc_s's comment that you don't need to check for DBNull.Value is wrong.

This is the generic method that I use to convert any object that might be a DBNull.Value:

public static T ConvertDBNull<T>(object value, Func<object, T> conversionFunction)
{
    return conversionFunction(value == DBNull.Value ? null : value);
}

usage:

var result = command.ExecuteScalar();

return result.ConvertDBNull(Convert.ToInt32);

shorter:

return command
    .ExecuteScalar()
    .ConvertDBNull(Convert.ToInt32);

Since I got an instance which isn't null and if I compared to DBNULL I got Operator '==' cannot be applied to operands of type 'string' and 'system.dbnull' exeption, and if I tried to change to compare to NULL, it simply didn't work ( since DBNull is an object) even that's the accepted answer.

I decided to simply use the 'is' keyword. So the result is very readable:

data = (item is DBNull) ? String.Empty : item

With a simple generic function you can make this very easy. Just do this:

return ConvertFromDBVal<string>(accountNumber);

using the function:

public static T ConvertFromDBVal<T>(object obj)
{
    if (obj == null || obj == DBNull.Value)
    {
        return default(T); // returns the default value for the type
    }
    else
    {
        return (T)obj;
    }
}