C# winform textbox, I have a textbox and want to enter decimals

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C# winform textbox, I have a textbox and want to enter decimals like .2 or .7.
I keep getting System.FormatException: 'Input string was not in a correct format.

C#

<pre> private void tb_Percentage_TextChanged(object sender, EventArgs e)
        {
           
            for (int i = 0; i < dataGridView1.Rows.Count; i++)

            {
                DataGridViewRow row = dataGridView1.Rows[i];
             
              decimal d = Convert.ToDecimal(dataGridView1.Rows[i].Cells[31].Value);
              //  tb_Percentage.Text = string.Format("{0:C2}", Convert.ToInt32(tb_Percentage.Text));
                decimal E = decimal.Parse(tb_Percentage.Text);
                decimal f = d * E;
                dataGridView1.Rows[i].Cells[32].Value = f.ToString("#.00");
                           
            }
        }

What I have tried:

//  tb_Percentage.Text = string.Format("{0:C2}", Convert.ToInt32(tb_Percentage.Text));

also tried Regular-Expressions

Posted 6-Mar-20 8:10am

Member 12349103

Updated 6-Mar-20 10:19am

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2 solutions

Solution 1

When trying to convert text to a number you will be better off using TryParse methods as you can determine if the conversion failed and take the appropriate action.

eg:

if (decimal.TryParse(stringvalue, out decimal myresult))
{
    // valid conversion comes here
    // myresult will contain a valid number on parse success
}
else
{
    // failed conversion comes here
    // myresult will be zero on parse failure
}

Posted 6-Mar-20 8:36am

Tony Hill

Updated 6-Mar-20 8:38am

v2

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MadMyche · Accepted Answer · 2020-03-06T10:19:00

Reading through the MS documentation and samples...
Convert Class (System) | Microsoft Docs[^]

FormatException
This occurs when the attempt to convert a string value to any other base type fails because the string is not in the proper format. The exception is thrown for the following conversions:
1. A string to be converted to a Boolean value does not equal Boolean.TrueString or Boolean.FalseString.
2. A string to be converted to a Char value consists of multiple characters.
3. A string to be converted to any numeric type is not recognized as a valid number

And if we look at the code block up the page, for the attempt to convert to Int

C#

int newInteger = 0;
try {
	System.Console.WriteLine("Enter an integer:");
	newInteger = System.Convert.ToInt32(System.Console.ReadLine());
}
//catch (System.ArgumentNullException) { ... }

catch (System.FormatException) {
  System.Console.WriteLine(

    "String does not consist of an optional sign followed by a series of digits."

  );
} 
//catch (System.OverflowException) {... }

"String does not consist of an optional sign followed by a series of digits."
It would appear that the only thing that may start a number is either a sign (for positive or negative) or a number.
The values you were provided in your question began with a decimal point, which may be why you are failing.

According to a couple of resources, the correct notation for a decimal value less than 1 is to begin with a "0" before the decimal point. I never really thought about it.
Numbers - Decimal Numbers - In Depth[^]
Writing a Decimal Number Less Than 1 Given its Name - Tutorialspoint[^]

Now that we know all of this... what should I do?
As the other answer states, you may want to try using the TryParse method

C#

decimal d = 0;
if (!Decimal.TryParse(dataGridView1.Rows[i].Cells[31].Value, out d) {
	// Parsing failed !
	// handle appropriately
}

Decimal.TryParse Method (System) | Microsoft Docs[^]

Another option would be to check the string first to see if the first character is a decimal point; and if so, prepend it with a "0"

C#

//decimal d = Convert.ToDecimal(dataGridView1.Rows[i].Cells[31].Value);
string Cell31 = dataGridView1.Rows[i].Cells[31].Value;
if (Cell31.Substring(0,1) == ".") { Cell31 = "0" + Cell31; }
decimal d = Convert.ToDecimal(Cell31);