C# Form Textbox string calculation with datatable()

Question

I just started with C# a week ago (programming with C++ like 2 1/2 years) and today I started my first form application. It is just a simple calculator.

So I have a form with the buttons 0-9; + ;- ;*;/ (and some more but they are not interesting for this problem) and one multiple line textbox. If the user press a button the specific symbol or digit is written directly to the output textbox. So the input part is working fine.

The problem is the calculation part. The code for this part is:

DataTable dt = new DataTable();
        double result = (double)dt.Compute(output.Text.ToString(), "");
        output.AppendText($" ={result.ToString()}");

If I run it I'm getting this error:

*System.InvalidCastException
  HResult=0x80004002
  Message=Specified cast is not valid.
  Source=SimpleCalculator
  StackTrace:
   at SimpleCalculator.Form1.result_Click(Object sender, EventArgs e) in C:\Users\Max\Documents\C#\Projects\SimpleCalculator\SimpleCalculator\Form1.cs:line 105
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.Run(Form mainForm)
   at SimpleCalculator.Program.Main() in*

So the interesting thing is ill changed it to int 32 (what's the error message is saying)

double result = (int32)dt.Compute(output.Text.ToString(), "");

it works fine until double values then it runs to an error (what is logic) but if I change it back to double its working.

double result = (double)dt.Compute(output.Text.ToString(), "");

I'm a bit confused about this because I didn't change anything in the code and the error comes sometimes back.

I mean yeah its working but not clean enough for me. I want to have an code without any chance to run to an error.

I'm using Visual Studio 2017 Community, is it because of VB or because of bad code?

Why can't you use `double`? If a calculation sometimes returns integers and sometimes doubles, use `double` because it works for both. `1+1=2` returns `int` and `1/1=1.0` returns `double` — Tim Schmelter, Oct 08 '18 at 12:50
Im new to visual studio (For c++ im using qt creator) where can i debug a special line of code? PS: Right now its working again i dont get it why this appear only sometimes without changing anything to this fucntion — Max, Oct 08 '18 at 12:51
for example in the quick watch window. But look at my edited comment, you see that you need `double` because that handles all calculations — Tim Schmelter, Oct 08 '18 at 12:52
Im using double but you see in the error message that he wants an type of int32 and how i said its working but if im getting a double value back of course the int32 produce an error. The error with the double is the long error message what only appears sometimes but actually annoying me . — Max, Oct 08 '18 at 12:54
Im using this code: DataTable dt = new DataTable(); double result = (double)dt.Compute(output.Text.ToString(), ""); string newout = result.ToString(); Im using doubles but sometimes its running to this error on top and i dont know why. — Max, Oct 08 '18 at 12:54

score 0 · Accepted Answer · answered Oct 08 '18 at 12:54

0

You may use the Convert class instead of casting. Example:

double result = Convert.ToDouble(dt.Compute(output.Text.ToString(), ""));

answered Oct 08 '18 at 12:54

kaffekopp

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score 0 · Answer 2 · answered Oct 08 '18 at 12:56

You need to use double. DataTable.Compute supports multiple operators and the result sometimes is an integer and sometimes a double. So use the greater type that works with always: double.

This returns int:

new DataTable().Compute("1+1", null)  // 2 int

and this returns double:

new DataTable().Compute("1/1", null) // 1.0 double

You can use System.Convert.To.Double:

System.Convert.ToDouble(new DataTable().Compute("1+1", null));

Operators, rules and syntax is explained here

score 0 · Answer 3 · edited Oct 09 '18 at 09:59

0

I know about the syntax of this function, that's not the problem; this error only occurs sometimes (by integer returns too).

I'll try now kaffekopps resolution and accept his answer. If the error comes back I'll reopen this question.

edited Oct 09 '18 at 09:59

halfer

19,824
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answered Oct 08 '18 at 13:05

Max

62
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score 0 · Answer 4 · answered Oct 08 '18 at 13:11

You are getting error because Datatable.Compute returns Object which contains text. In C#, casting Text to Number wont compile because compiler cant convert text to number. Text may not contain only numbers but any character. You will have to use Convert.To()

If you are interested into this problematic you can read question about difference between casting and using Convert.

In you code, if you change line

 double result = (double)dt.Compute(output.Text.ToString(), "");

into

 double result = Convert.ToDouble(dt.Compute(output.Text.ToString(), ""));

it should works fine.

C# Form Textbox string calculation with datatable()

4 Answers4