Language invariant Double.ToString()

Use:

string networkMsg = "command " + value.ToString(CultureInfo.InvariantCulture);

or:

string networkMsg = string.Format(CultureInfo.InvariantCulture, "command {0}", value);

This needs using System.Globalization; in the top of your file.

Note: If you need full precision, so that you can restore the exact double again, use the Format solution with the roundtrip format {0:R}, instead of just {0}. You can use other format strings, for example {0:N4} will insert thousands separators and round to four dicimals (four digits after the decimal point).

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Since C# 6.0 (2015), you can now use:

string networkMsg = FormattableString.Invariant($"command {value}");

Specify the invariant culture as the format provider:

value.ToString(CultureInfo.InvariantCulture);

The . in the format specifier "0.0" doesn't actually mean "dot" - it means "decimal separator" - which is , in France and several other European cultures. You probably want:

value.ToString(CultureInfo.InvariantCulture)

or

value.ToString("0.0", CultureInfo.InvariantCulture)

For info, you can see this (and many other things) by inspecting the fr culture:

var decimalSeparator = CultureInfo.GetCultureInfo("fr")
            .NumberFormat.NumberDecimalSeparator;