how to convert six decimal no to 3 decimal in C#

Use ToString method with "Fx" and replace x for the number of decimals you want:

double val = 1222.34555566;
//get 4 decimals
string txt = val.ToString("F4");

Try this:

String.Format("{0:0.0000}", 1222.34555566); 

...and check out this link:
http://www.csharp-examples.net/string-format-double/[^]

Good luck!

I want to convert 1222.34555566 to 1222.3455

What you have asked is you want to convert Six decimal place to Three and the thing that you have mentioned above is different. However considering your given thing you can Get result like this by trying below code.

double Num = 1222.34555566;
string nNum = Num.ToString("#0.0000");
You will get nNum = 1222.3455

I hope this is what you want.