Convert Decimal to Double?

Question

I want to use a track-bar to change a form's opacity.

This is my code:

decimal trans = trackBar1.Value / 5000;
this.Opacity = trans;

When I build the application, it gives the following error:

Cannot implicitly convert type 'decimal' to 'double'.

I tried using trans and double but then the control doesn't work. This code worked fine in a past VB.NET project.

Answer 1

It sounds like this.Opacity is a double value, and the compiler doesn't like you trying to cram a decimal value into it.

Answer 2

this.Opacity = trackBar1.Value / 5000d;

Answer 3

Since Opacity is a double value, I would just use a double from the outset and not cast at all, but be sure to use a double when dividing so you don't loose any precision

Opacity = trackBar1.Value / 5000.0;

Answer 4

Your code worked fine in VB.NET because it implicitly does any casts, while C# has both implicit and explicit ones.

In C# the conversion from decimal to double is explicit as you lose accuracy. For instance 1.1 can't be accurately expressed as a double, but can as a decimal (see "Floating point numbers - more inaccurate than you think" for the reason why).

In VB the conversion was added for you by the compiler:

decimal trans = trackBar1.Value / 5000m;
this.Opacity = (double) trans;

That (double) has to be explicitly stated in C#, but can be implied by VB's more 'forgiving' compiler.

Answer 5

In my opinion, it is desirable to be as explicit as possible. This adds clarity to the code and aids your fellow programmers who may eventually read it.

In addition to (or instead of) appending a .0 to the number, you can use decimal.ToDouble().

Here are some examples:

// Example 1
double transperancy = trackBar1.Value/5000;
this.Opacity = decimal.ToDouble(transperancy);

// Example 2 - with inline temp
this.Opacity = decimal.ToDouble(trackBar1.Value/5000);

Answer 6

You should use 5000.0 instead of 5000.