How can I check if a string is a number?

Starting with C# 7.0, you can declare the out variable in the argument list of the method call, rather than in a separate variable declaration. This produces more compact, readable code, and also prevents you from inadvertently assigning a value to the variable before the method call.

bool isDouble = double.TryParse(yourString, out double result);

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/out-parameter-modifier