C# How to treat a string variable as interpolated string?

Question

the interpolated string is easy, just a string lead with $ sign. But what if the string template is coming from outside of your code. For example assume you have a XML file containing following line:

<filePath from="C:\data\settle{date}.csv" to="D:\data\settle{date}.csv"/>

Then you can use LINQ to XML read the content of the attributes in.

//assume the ele is the node <filePath></filePath>
string pathFrom = ele.Attribute("from").value;
string pathTo = ele.Attibute("to").value;
string date = DateTime.Today.ToString("MMddyyyy");

Now how can I inject the date into the pathFrom variable and pathTo variable?

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If I have the control of the string itself, things are easy. I can just do var xxx=$"C:\data\settle{date}.csv";But now, what I have is only the variable that I know contains the placeholder date

Answer 1

String interpolation is a compiler feature, so it cannot be used at runtime. This should be clear from the fact that the names of the variables in the scope will in general not be availabe at runtime.

So you will have to roll your own replacement mechanism. It depends on your exact requirements what is best here.

If you only have one (or very few replacements), just do

output = input.Replace("{date}", date);

If the possible replacements are a long list, it might be better to use

output = Regex.Replace(input, @"\{\w+?\}", 
    match => GetValue(match.Value));

with

string GetValue(string variable)
{
    switch (variable)
    {
    case "{date}":
        return DateTime.Today.ToString("MMddyyyy");
    default:
        return "";
    }
}

If you can get an IDictionary<string, string> mapping variable names to values you may simplify this to

output = Regex.Replace(input, @"\{\w+?\}", 
    match => replacements[match.Value.Substring(1, match.Value.Length-2)]);

Answer 2

You can't directly; the compiler turns your:

string world = "world";
var hw = $"Hello {world}"

Into something like:

string world = "world";
var hw = string.Format("Hello {0}", world);

(It chooses concat, format or formattablestring depending on the situation)

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You could engage in a similar process yourself, by replacing "{date" with "{0" and putting the date as the second argument to a string format, etc.