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:

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

//assume the ele is the node 
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?

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

user_2142950 · Answer

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 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)]);

user_8795884 · Answer

SOLUTION 1:
If you have the ability to change something on xml template change {date} to {0}.

Then you can set the value of that like this.

var elementString = string.Format(element.ToString(), DateTime.Now.ToString("MMddyyyy"));

Output:

SOLUTION 2:
If you can't change the xml template, then this might be my personal course to go.

Set the placeholder like this.

element.Attribute("to").Value = element.Attribute("to").Value.Replace("{date}", DateTime.Now.ToString("MMddyyyy"));
element.Attribute("from").Value = element.Attribute("from").Value.Replace("{date}", DateTime.Now.ToString("MMddyyyy"));

Output:

I hope it helps. Kind regards.

user_1410664 · Answer

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)

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.

user_2301887 · Answer

If you treat your original string as a user-input string (or anything that is not processed by the compiler to replace the placeholder, then the question is simple - just use String.Replace() to replace the placehoder {date}, with the value of the date as you wish. Now the followup question is: are you sure that the compiler is not substituting it during compile time, and leaving it untouched for handling at the runtime?

user_7337370 · Answer

String interpolation allows the developer to combine variables and text to form a string.

Example Two int variables are created: foo and bar.

int foo = 34;
int bar = 42;

string resultString = $"The foo is {foo}, and the bar is {bar}.";

Console.WriteLine(resultString);

Output:

The foo is 34, and the bar is 42.

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

5 Answers