How to call a function based on string value in C#

Use the double.TryParse Method.

The method attempts to convert a string to a double and if it fails it returns false.

If I'm understanding you correctly, this is what you want:

private static double ConditionalConvertToDouble(string str1, string str2) {
    double converted;
    if (double.TryParse(str2, out converted)) {
        // str2 can be converted to a double, so return str1 converted to a double.
        return Convert.ToDouble(str1);
    } else {
        // I'm throwing an exception here if str1 cannot be converted to a double; you 
        // might want to do something different.
        throw new ArgumentException("str1 cannot be converted to a double");
    }
}

Call the method like this:

var d = ConditionalConvertToDouble("11", "22");

double result;
Double.TryParse(str1, out result);

if str1 is double, result will have its value, otherwise result will have 0. It will not throw any exception.

Hope it helps.

The following method will attempt to get the double value from "str1" if and only if "str2" also represents a double. If either of the numbers are not doubles, it will return double.NaN.

This method avoids throwing exceptions, but still allows you to check if you have a valid number.

public double GetFirstDoubleIfSecond(string str1, string str2)
{
    double myDouble;

    if(double.TryParse(str2, out myVal) && double.TryParse(str1, out myVal))
    {
        return myDouble
    }

    return double.NaN;
}

If you are expecting NaN values in your string, then you can use the following method, which will throw an exception if either of the strings are not doubles. Otherwise, it will return the string value represented in "str1".

public double GetFirstDoubleIfSecondWithExceptions(string str1, string str2)
{
    double.Parse(str2);

    return double.Parse(str1);
}

I think that switch-case operator should help to you. You should to specify some cases depending on one of your input string and make right decision. Your method should return object, if you really don't know which type exactly will return your method.

public object MyMethod(string str1, string str2)
{
    switch(something)
    {
            case 1:
            case 2:
                return double.Parse(str1);
            break;
            case 3:
            case 4:
                return int.Parse(str1);
            break;
            default
                return null;
    }
}

where something is the criterion you should to specify.

After that System.Object.GetType() method can help you to detirmine which type was returned by your method.