How to get pass this error "Value cannot be null. Parameter name: entity"

        private void button1_Click(object sender, EventArgs e)
    {

        DataClasses1DataContext dc = new DataClasses1DataContext();
        var rec = dc.reportsSents.FirstOrD开发者_如何学Pythonefault();
        int rowCount = dc.reportsSents.Count();


        if (rec != null)
            {

            for(int i = 0; i <= rowCount;)
                {


                    var matchedCaseNumber = (from CaseNumberKey in dc.GetTable<reportsSent>()
                                     select CaseNumberKey).FirstOrDefault();

                    (new MyReportRenderer()).RenderTest(Convert.ToString(matchedCaseNumber));

                    dc.reportsSents.DeleteOnSubmit(matchedCaseNumber);
                    dc.SubmitChanges();

                    i = (i +1);

                } 

             }  

When the code above is executed I get this error:

Value cannot be null. Parameter name: entity

and matchedCaseNumber is highlighted in this line:

dc.reportsSents.DeleteOnSubmit(matchedCaseNumber);

What does this mean and why am I getting it. What I am trying to do is pass MatchedCaseNumber to the method and then delete it after the method is executed and then step through the table. Any help would be appreciated.

Thanks.

You have

var matchedCaseNumber = (...).FirstOrDefault();

dc.reportsSents.DeleteOnSubmit(matchedCaseNumber);

So what if matchedCaseNumber is null? The OrDefault makes that possible.

Change that 2nd line into:

if (matchedCaseNumber != null)
    dc.reportsSents.DeleteOnSubmit(matchedCaseNumber);

Also,

int rowCount = dc.reportsSents.Count();

for(int i = 0; i <= rowCount;)
{
   ...
   i = (i +1);    
}

should probably become:

for(int i = 0; i < rowCount; i += 1)  // note: NOT <=
{
    ...    
}

You are looping rowCount+1 times. And there really is no point in moving the increment of i out of the loop statement. Don't confuse/hinder opitimizers.

matchedCaseNumber is probably null when passed to DeleteOnSubmit

if(matchedCaseNumber != null)
{
  dc.reportsSents.DeleteOnSubmit(matchedCaseNumber);
  dc.SubmitChanges();
}

EDIT: If I understand correctly what you're trying to do, this would be better:

private void button1_Click(object sender, EventArgs e)
{

    DataClasses1DataContext dc = new DataClasses1DataContext();

    foreach (var item in dc.reportsSents)
    {
        (new MyReportRenderer()).RenderTest(Convert.ToString(item));
        dc.reportsSents.DeleteOnSubmit(item);
    }
    dc.SubmitChanges();
}

It means that matchCaseNumber is null.

When you create matchCaseNumber you use .FirstOrDefault() which returns the first instance or the default value for that type (which is null for reference types).

In other words, your query:

from CaseNumberKey in dc.GetTable<reportsSent>()
select CaseNumberKey

appears to be returning no data.