InvalidCastException when getting first result from LINQ query
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Thread: InvalidCastException when getting first result from LINQ query

  1. #1
    Join Date
    Aug 2006
    Posts
    132

    Question InvalidCastException when getting first result from LINQ query

    Greetings!

    I am a complete newbie at LINQ. I am trying to get my first simple little application using it to run. It's built in Visual Studio 2008, using DevArt's LinqConnect product to talk to a PostgreSQL database. Here's the function:

    Code:
    		private void dgvCharges_CellClick(object sender, DataGridViewCellEventArgs e)
    		{
    			long chargeNumber = Convert.ToInt32(dgvCharges[e.ColumnIndex, e.RowIndex].Value);
    			var oneChargeSet = from charge in m_context.Charges
    							where charge.Charge1 == chargeNumber
    							select charge;
    			int chargeCount = oneChargeSet.Count();
    			var oneCharge = oneChargeSet.First();                      
    		}
    In the debugger, I see that chargeCount is 1, as expected. I believe that the First() function should return an object that contains all of the data members of the charge table in the database: Charge1, BuiltDate, LoadedDate, Cycle, and everything else. This code compiles and starts nicely. But when I invoke the First() method, I get an InvalidCastException. I get the same expression when I try to give First an explicit type:

    Code:
    		private void dgvCharges_CellClick(object sender, DataGridViewCellEventArgs e)
    		{
    			long chargeNumber = Convert.ToInt32(dgvCharges[e.ColumnIndex, e.RowIndex].Value);
    			var oneChargeSet = from charge in m_context.Charges
    							where charge.Charge1 == chargeNumber
    							select charge;
    			int chargeCount = oneChargeSet.Count();
    			var oneCharge = oneChargeSet.First<AnnealContext.Charge>();
    		}
    Obviously, I don't understand something basic about LINQ. How do I get this to work?

    Thanks very much!

    RobR

  2. #2
    Join Date
    Aug 2006
    Posts
    132

    Re: InvalidCastException when getting first result from LINQ query

    Update: If I do not specify an explicit list of fields, I can pass the query object into a data view control, and the control will display the contents of the table as expected. If I want to use the results of the query in my application, I can include an explicit list of fields to return, and it will work. That's probably a good idea anyway. I remain curious, though, why I can't get the simple first form to work.

    Here's what's working for me:
    Code:
    			var oneChargeSet = from charge in m_context.Charges
    							where charge.Charge1 == chargeNumber
    							select new 
    							{ 
    								charge.Charge1, 
    								charge.Loaded_Date,
    								charge.Purge_Date,
    								charge.Standby_Date,
    								charge.Fire_Date,
    								charge.Off_Date,
    								charge.Cool_Date,
    								charge.Cold_Date,
    								charge.Unloaded_Date
    							};
    			
    			var oneCharge = oneChargeSet.First();

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