Limiting the decimal point into 4 digits in vb.net

I can limit the decimal point when I declare the variable as decimal though I can't when I declare it as a string. Because I need it in string datatype.

Dim result As Decimal
result = ComputationResult
Textbox1.Text = result.ToString("N4")

^^ The above code works

But this doesn't :

Dim result As String
result = ComputationResult
Textbox1.Text = result.ToString("N4")

If ComputationResult is a Decimal, when result is declared as a string you have an implicit conversion from Decimal to String on this line using the default format:

 result = ComputationResult

So now result is already a string, and you lost the opportunity to use numeric format strings. The "N4" in the ToString() call has no meaning:

 Textbox1.Text = result.ToString("N4")

It's trying to use this overload, where N4 will fail to match any format provider... but it doesn't matter, because the linked documentation says this argument is ignored anyway:

> provider is reserved, and does not currently participate in this operation.
>
> Because this method simply returns the current string unchanged, there is no need to call it directly.

On the other hand, if ComputationResult is a String, the version where result is a Decimal forces an implicit conversion from String to Decimal, which allows allows the result.ToString("N4") call to use this overload that knows about format strings.

What we learn from this is data types matter! Better developers are aware of the data type in play at every level of their expressions, and know how to use the appropriate types in the appropriate places, while avoiding needless conversions.

Remember, thanks to cultural/internationalization issues, converting between a number (or date) and a string is FAR slower and more error-prone than you expect. It's not the quick or simple operation we'd like it to be, but rather something to avoid. The best plan is generally to keep a value in a number type for as long as possible, and only convert to string for output at the last possible moment.

Really, the fact the result = ComputationResult assignment compiles for both samples tells me you have both Option Strict and Option Infer turned off, and that's REALLY BAD! You should turn at least one of those on, and then go correct the resulting errors in your program until the code compiles again. You will be SO MUCH BETTER OFF for this work: your code will be faster, MUCH less likely to crash, and it will actually make you a faster developer once you learn to adapt to the constraint.

Since we don't know that actual type of ComputationResult, there are two possible correct versions of the code. Either you want this if it's a String:

Textbox1.Text = Decimal.Parse(ComputationResult).ToString("N4")

or this if it's a Decimal:

Textbox1.Text = ComputationResult.ToString("N4")

And really, if ComputationResult is defined in a sane way it will already be a Decimal.

Finally, it looks like ComputationResult might be a method with no arguments. While VB.Net still supports calling methods without parentheses for the argument list, you should not do that. For .Net, ALWAYS use parentheses when calling your methods.

Textbox1.Text = ComputationResult().ToString("N4")

But if it's not the case ComputationResult is a method you can ignore this last item.

If your cumputationResult is already a string, convert it to a decimal using Decimal.Parse, then do the number formatting on it.

Dim cumputationResult = "1.234567"
Textbox1.Text = Decimal.Parse(cumputationResult).ToString("N4")