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When converting a string to double the output is whitout leading zeros

I have a textbox where i get my string value from.
When using
tbValue = (Convert.ToDouble(tbValue.text) * -1.0).toString();
the output is allways without the leading zeros.
when the value is 0,8 it returns -8 in the textbox.
Is there a way to keep the leading zero when using Convert.ToDouble?

First, your error is not about "leading zeroes". A double is a value, values have no surplus leading zeroes (like in 003.3) - those are added in formatting the output.
If your input of "0.8" is recognized as "8" that is not a leading zero issue, it is a decimal separator issue - the "ToDouble" part is assuming - likely - that there is a "." as decimal separator, so the "," is ignored. The number is only a proper number if parsed with the correct locale for your system (default is to the system language). You can give an explicit locale in Parsing, check all overloads of ToDouble.

Are you using "," ?
It works with "."
string input = "0.8";
string tbValue = (Convert.ToDouble(input) * -1.0).ToString();
https://dotnetfiddle.net/XAsNhe

This work
string tbValue = (Convert.ToDouble(tbValue.Text.Replace(",","."), CultureInfo.InvariantCulture) * -1.0).ToString();

Double to string with mandatory decimal point

This is probably dumb but it's giving me a hard time. I need to convert/format a double to string with a mandatory decimal point.
1 => 1.0
0.2423423 => 0.2423423
0.1 => 0.1
1234 => 1234.0
Basically, I want to output all decimals but also make sure the rounded values have the redundant .0 too. I am sure there is a simple way to achieve this.

Use double.ToString("N1"):
double d1 = 1d;
double d2 = 0.2423423d;
double d3 = 0.1d;
double d4 = 1234d;
Console.WriteLine(d1.ToString("N1"));
Console.WriteLine(d2.ToString("N1"));
Console.WriteLine(d3.ToString("N1"));
Console.WriteLine(d4.ToString("N1"));
Demo
Standard Numeric Format Strings
The Numeric ("N") Format Specifier
Update
(1.234).ToString("N1") produces 1.2 and in addition to removing additional decimal digits, it also adds a thousands separator
Well, perhaps you need to implement a custom NumberFormatInfo object which you can derive from the current CultureInfo and use in double.ToString:
var culture = CultureInfo.CurrentCulture;
var customNfi = (NumberFormatInfo)culture.NumberFormat.Clone();
customNfi.NumberDecimalDigits = 1;
customNfi.NumberGroupSeparator = "";
Console.WriteLine(d1.ToString(customNfi));
Note that you need to clone it since it's readonly by default.
Demo

There is not a built in method to append a mandatory .0 to the end of whole numbers with the .ToString() method, as the existing formats will truncate or round based on the number of decimal places you specify.
My suggestion is to just roll your own implementation with an extension method
public static String ToDecmialString(this double source)
{
if ((source % 1) == 0)
return source.ToString("f1");
else
return source.ToString();
}
And the usage:
double d1 = 1;
double d2 = 0.2423423;
double d3 = 0.1;
double d4 = 1234;
Console.WriteLine(d1.ToDecimalString());
Console.WriteLine(d2.ToDecimalString());
Console.WriteLine(d3.ToDecimalString());
Console.WriteLine(d4.ToDecimalString());
Results in this output:
1.0
0.2423423
0.1
1234.0

You could do something like this: if the number doesn't have decimal points you can format its output to enforce one decimal 0 and if it has decimal places, just use ToString();
double a1 = 1;
double a2 = 0.2423423;
string result = string.Empty;
if(a1 - Math.Floor(a1) >0.0)
result = a1.ToString();
else
result = a1.ToString("F1");
if (a2 - Math.Floor(a2) > 0.0)
result = a2.ToString();
else
result = a2.ToString("F1");
When you use "F" as formatting, the output won't contain thousands separator and the number that follows it specifies the number of decimal places.

Use ToString("0.0###########################").
It does work. I found it in duplicate of your question decimal ToString formatting which gives at least 1 digit, no upper limit

Double provides a method ToString() where you can pass an IFormatProvider-object stating how you want your double to be converted.
Additionally, it should display trailing 0 at all costs.
value = 16034.125E21;
// Display value using the invariant culture.
Console.WriteLine(value.ToString(CultureInfo.InvariantCulture));
// Display value using the en-GB culture.
Console.WriteLine(value.ToString(CultureInfo.CreateSpecificCulture("en-GB")));
// Display value using the de-DE culture.
Console.WriteLine(value.ToString(CultureInfo.CreateSpecificCulture("de-DE")));
// This example displays the following output to the console:
// -16325.62015
// -16325.62015
// -16325,62015
// 1.6034125E+25
// 1.6034125E+25
// 1,6034125E+25
Here is the documentation from MSDN.

You can cast to string and then appen ".0" if there was no decimal point given
string sValue=doubleValue.ToString();
if(!sValue.Contains('.'))
sValue+=".0";
EDIT:
As mentioned in the comments '.' may not be the decimal seperator in the current culture. Refer to this article to retrieve the actual seperator if you want make your code save for this case.

Convert a string to double

I need help converting a string to double with 7 decimals. I have a string "00000827700000" and need it converted to 82.77
Tried using String.Format() with {0:N7} without success.

It looks like you could use:
decimal x = decimal.Parse(text.Substring(0, 7) + "." +
text.Substring(7),
CultureInfo.InvariantCulture);
That would actually parse it to 82.7700000, as decimal preserves trailing zeroes (to an extent) but maybe that's good enough? It not, you could change the second argument to
text.Substring(7).TrimEnd('0')
Note that I'd strongly recommend you to at least consider using decimal instead of double. You haven't explained what this value represents, but if it's stored as decimal figures which you need to preserve, it smells more like a decimal to me.

Based on the edit, it could be simplified as
var text = "00000827700000";
var x = decimal.Parse(text, CultureInfo.InvariantCulture) / 10000000;
Console.Write(String.Format("{0:N7}", x));

Double to String keeping trailing zero

I've tried converting the double value into a string and using the Replace() method
to replace the ',' to '.'.
This works well but only when the trailing digits are not zero, I need zeros in my string, even if the value is 1234.0. This worked well for the decimal values. I have tried to convert the double to decimal but I lose the decimal digits if there are zeros.
I know I'm missing something. I would be grateful for some suggestions.

This would depend on the language. An example in C#
d.ToString("0.00");
Would produce a double with 2 decimal places nomatter the values (zero or otherwise).

If this is in Java, check out the NumberFormat class's setMinimumFractionDigits() method.
Example:
double d1 = 2.5;
double d2 = 5.0;
NumberFormat nf = NumberFormat.getInstance();
nf.setMinimumFractionDigits(2);
String d1s = nf.format(d1);
String d2s = nf.format(d2);
System.out.println("d1s: " + d1s + " and d2s: " + d2s);
produces
d1s: 2.50 and d2s: 5.00

...and in Fortran, you could do something like: :-)
write(*,110) x
110 format (F5.3)
(guess we really have to know what language is being used...)

parsing float into string

I have a number like so: 4.47778E+11
Can anyone give me a way of converting that into its number representation easily in c#?
Thanks

string s = "4.47778e+11";
double d = double.Parse(s);
or
string s = "4.47778e+11";
if (double.TryParse(s,out d))
{
// number was parsed correctly
}
or for internationalization
double.Parse("4.47778e+11", System.Globalization.NumberStyles.Float, System.Globalization.CultureInfo.InvariantCulture);

Try this MSDN thread. It's called scientific notation by the way, and a quick google normally solves simple issues.
That's assuming you mean parsing from a string to a float, your question & title are conflicting

Use
float num = Convert.ToFloat(Convert.ToDouble(s));
But you still lose precision, floats are only precise to 7 digits, so you're better off using just the Convert.ToDouble() (precise to 15 or so digits), so you won't lose any digits in your example.

Use Convert:
double value = Convert.ToDouble("4.47778E+11");