Help with understanding this code and rewriting code

I found this code and I am able to use it in another program with no problem.  It gets me the total of the line items checked.

Now I need to change it but I haven't seen code written like this. Can someone help me read it?  I get the basics but I need help adding to it.

public static decimal GetCheckedSum(this DataGridView sender, string CheckedColumn, string AmountColumnName)
        {
            return
                (
                    from Rows in sender.Rows.Cast<DataGridViewRow>()
                    where Convert.ToBoolean(Rows.Cells[CheckedColumn].Value) == true
                    select Convert.ToDecimal(Rows.Cells[AmountColumnName].Value)
                ).Sum();
        }

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As you can see below if there is a BillAmt then there is nothing in the PaymentAmt Column.  When I run the above code I get an error because there is no number in the PaymentAmt column.

I have a grid that displays the amt of a bill or a payment.  If the user clicks on the checkbox I want to get the Total for the Bills and Payments.  Here is how I have the grid laid out.  If it is all messy I have attached a screen print to see.

CheckBox         ID                   BillAmt            PaymentAmt
                           45                    4,500.00                      
                           46                       200.00    
                           47                                               4,000.00
                           48                    2,000.00
                           49                                               5,000.00


I would like to add an IF Statement or something similar
 if (BillAmt == "")
{
    BillAmt = 0;
}

OR in the SELECT portion change it to this

select ISNULL(Convert.ToDecimal(Rows.Cells[AmountColumnName].Value), 0, Convert.ToDecimal(Rows.Cells[AmountColumnName].Value))

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Doc3.docx
huerita37Asked:
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zephyr_hex (Megan)DeveloperCommented:
Given your event method:
public static decimal GetCheckedSum(this DataGridView sender, string CheckedColumn, string AmountColumnName)
        {
            return
                (
                    from Rows in sender.Rows.Cast<DataGridViewRow>()
                    where Convert.ToBoolean(Rows.Cells[CheckedColumn].Value) == true
                    select Convert.ToDecimal(Rows.Cells[AmountColumnName].Value)
                ).Sum();
        }

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This method returns a decimal value based on the results of the Linq query:

from Rows in sender.Rows.Cast<DataGridViewRow>()
where Convert.ToBoolean(Rows.Cells[CheckedColumn].Value) == true
select Convert.ToDecimal(Rows.Cells[AmountColumnName].Value)

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That Linq query is looking at the rows in the gridview, where the checkbox has been selected, and then summing the value of the amount column for those rows.

The amount column is dependent on what's passed to the method.

This doesn't answer your entire question.  But hopefully it explains the method and how it's working.

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huerita37Author Commented:
Thank you very much.  You pointed me in the right direction.  This is what I ended up with and is  working:

public static decimal GetCheckedSum(this DataGridView sender, string ColumnName, string AmountColumnName)
        {
            return
                (
                    from Rows in sender.Rows.Cast<DataGridViewRow>()
                    where Convert.ToBoolean(Rows.Cells[ColumnName].Value) == true
                    && Convert.ToString(Rows.Cells[AmountColumnName].Value) != ""
                    select Convert.ToDecimal(Rows.Cells[AmountColumnName].Value)
                ).Sum();
        }
funwithdotnetCommented:
It loos like it's not accounting for null values.

How about something like:
3: select Convert.ToDecimal(Rows.Cells[AmountColumnName].Value = null : 0 : Rows.Cells[AmountColumnName].Value)

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or
2: where Convert.ToBoolean(Rows.Cells[CheckedColumn].Value) == true && Rows.Cells[AmountColumnName].Value != null

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I'm not  a LINQ or C# person, so my syntax might be off a bit.
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