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

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?
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

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)]);

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.

SOLUTION 1:
If you have the ability to change something on xml template change {date} to {0}.
<filePath from="C:\data\settle{0}.csv" to="D:\data\settle{0}.csv" />
Then you can set the value of that like this.
var elementString = string.Format(element.ToString(), DateTime.Now.ToString("MMddyyyy"));
Output: <filePath from="C:\data\settle08092020.csv" to="D:\data\settle08092020.csv" />
SOLUTION 2:
If you can't change the xml template, then this might be my personal course to go.
<filePath from="C:\data\settle{date}.csv" to="D:\data\settle{date}.csv" />
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: <filePath from="C:\data\settle08092020.csv" to="D:\data\settle08092020.csv" />
I hope it helps. Kind regards.

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?

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.

Interpolation in the middle of a string

I want to interpolation in the middle of a string, but I cannot use String.Format, because the string contains {} curly brackets.
This is what I have tried so far:
string code = "(function(){var myvariable = $"{variableToBeInterpolated}"});"
Returns: ) expected ; expected
Edit: I tried the following snippet, the interpolation is now working, but the code is not executed in the browser.
"(function(){var myvariable=" + $"{myvariable c#}" + ")});"

General Information
With C# version 6, extensive string interpolation capabilities have been added to the language.
String interpolation provides a more readable and convenient syntax to
create formatted strings than a string composite formatting feature.
To solve your problem, please have a look at the special characters section of the documentation, which reads the following:
To include a brace, "{" or "}", in the text produced by an
interpolated string, use two braces, "{{" or "}}".
Example
var variable = 10;
var code = $"(function() {{ var myVariable = {variable} }});";
Console.WriteLine(code);
Output: (function() { var myVariable = 10 });

Have you tried:
string someVariable = "test";
string code = $"(function()'{{var myvariable = ({someVariable} in c#)}});"
Using $ in C# is string interpolation. Added in C# 6
learn.microsoft.com

Why do you want to interpolate in the middle, rather put $ in front. And for { you need to use {{. (the same applies for })
string code = $"(function(){{ var myvariable = {myvariable} }});";

Interpolate a string retrieved from a file? [duplicate]

I can do this:
var log = string.Format("URL: {0}", url);
or even like this
var format = "URL: {0}";
...
var log = string.Format(format, url);
I have a format defined somewhere else and use the format variable, not inline string.
In C# 6, this is seems impossible:
var format = $"URL: {url}"; // Error url does not exist
...
var url = "http://google.com";
...
var log = $format; // The way to evaluate string interpolation here
Is there anyway to use string interpolation with variable declared earlier?
C# 6 seems interpolate the string inline during compile time. However consider using this feature for localization, define a format in config or simply having a format const in a class.

No, you can't use string interpolation with something other than a string literal as the compiler creates a "regular" format string even when you use string interpolation.
Because this:
string name = "bar";
string result = $"{name}";
is compiled into this:
string name = "bar";
string result = string.Format("{0}", name);
the string in runtime must be a "regular" format string and not the string interpolation equivalent.
You can use the plain old String.Format instead.

One approach to work around that would be to use a lambda containing the interpolated string. Something like:
Func<string, string> formatter = url => $"URL: {url}";
...
var googleUrl = "http://google.com";
...
var log = formatter(googleUrl);
In C# 7.0, you could use a local function instead of a lambda, to make the code slightly simpler and more efficient:
string formatter(string url) => $"URL: {url}";
...
var googleUrl = "http://google.com";
...
var log = formatter(googleUrl);

String interpolation is not library, but a compiler feature starting with C# 6.
The holes are not names, but expressions:
var r = new Rectangle(5, 4);
var s = $"Area: {r.Width * r.Heigh}":
How would you do that for localization, as you intend to?
Even r only exists at compile time. In IL it's just a position on the method's variable stack.
I've done what you intend to do for resources and configuration files.
Since you can only have a finite set of "variables" to substitute, what I did was have an array (or dictionary, if you prefer) and use a regular expression to replace the names in the holes with its index. What I did even allowed for format specifiers.

This is supposed to be a comment to the answer from i3arnon but I do not have the reputation :-( :
But for those who come to this old thread, in string.Format the format can be a variable:
string name = "bar";
string format = "{0}";
string result = string.Format(format, name);
works.

More of an idea as opposed to an answer.
For the example shown in the question, you can do the following.
var format = "URL: ";
...
var url = "http://google.com";
...
var result= $"{format} {url}";
I have an actual project where I have to do something like this a lot:
var label = "Some Label";
var value = "SomeValue";
//both label & value are results of some logic
var result = $"{label}: {value}";

You can with the right Nuget package: https://www.nuget.org/packages/InterpolatedStringFormatter
var mystring = "a thing(and something {other})";
Console.WriteLine(mystring.Interpolate("else"));
Outputs:
a thing(and something else)

It seems that you can do it like this:
var googleUrl = "http://google.com";
var url = $"URL: {googleUrl}";
System.Console.WriteLine(url);
You can check for more details in https://msdn.microsoft.com/en-us/library/dn961160.aspx

Using variables inside strings

In PHP I can do the following:
$name = 'John';
$var = "Hello {$name}"; // => Hello John
Is there a similar language construct in C#?
I know there is String.Format(); but I want to know if it can be done without calling a function/method on the string.

In C# 6 you can use string interpolation:
string name = "John";
string result = $"Hello {name}";
The syntax highlighting for this in Visual Studio makes it highly readable and all of the tokens are checked.

This functionality is not built-in to C# 5 or below.
Update: C# 6 now supports string interpolation, see newer answers.
The recommended way to do this would be with String.Format:
string name = "Scott";
string output = String.Format("Hello {0}", name);
However, I wrote a small open-source library called SmartFormat that extends String.Format so that it can use named placeholders (via reflection). So, you could do:
string name = "Scott";
string output = Smart.Format("Hello {name}", new{name}); // Results in "Hello Scott".
Hope you like it!

Use the following methods
1: Method one
var count = 123;
var message = $"Rows count is: {count}";
2: Method two
var count = 123;
var message = "Rows count is:" + count;
3: Method three
var count = 123;
var message = string.Format("Rows count is:{0}", count);
4: Method four
var count = 123;
var message = #"Rows
count
is:{0}" + count;
5: Method five
var count = 123;
var message = $#"Rows
count
is: {count}";

Up to C#5 (-VS2013) you have to call a function/method for it. Either a "normal" function such as String.Format or an overload of the + operator.
string str = "Hello " + name; // This calls an overload of operator +.
In C#6 (VS2015) string interpolation has been introduced (as described by other answers).

I saw this question and similar questions and I preferred to use a built-in method for the problem of using a dictionary of values to fill-in placeholders in a template string. Here's my solution, which is built on the StringFormatter class from this thread:
public static void ThrowErrorCodeWithPredefinedMessage(Enums.ErrorCode errorCode, Dictionary<string, object> values)
{
var str = new StringFormatter(MSG.UserErrorMessages[errorCode]) { Parameters = values};
var applicationException = new ApplicationException($"{errorCode}", new ApplicationException($"{str.ToString().Replace("#","")}"));
throw applicationException;
}
where the message exists in a Dictionary that is not in the caller, but the caller only has access to Enums.ErrorCode, and can build an argument array and send it to the above method as argument.
assuming we have the value of MSG.UserErrorMessages[errorCode] is originally
"The following entry exists in the dump but is not found in #FileName: #entryDumpValue"
The result of this call
var messageDictionary = new Dictionary<string, object> {
{ "FileName", sampleEntity.sourceFile.FileName}, {"entryDumpValue", entryDumpValue }
};
ThrowErrorCodeWithPredefinedMessage(Enums.ErrorCode.MissingRefFileEntry, messageDictionary);
is
The following entry exists in the dump but is not found in cellIdRules.ref: CellBand = L09
The only restriction to this approach is to avoid using '#' in the contents of any of the passed values.

you can define a variable as string like this in C#
var varName= data.Values["some var"] as string;

Is it possible to include a C# variable in a string variable without using a concatenator?

Does .NET 3.5 C# allow us to include a variable within a string variable without having to use the + concatenator (or string.Format(), for that matter).
For example (In the pseudo, I'm using a $ symbol to specify the variable):
DateTime d = DateTime.Now;
string s = "The date is $d";
Console.WriteLine(s);
Output:
The date is 4/12/2011 11:56:39 AM
Edit
Due to the handful of responses that suggested string.Format(), I can only assume that my original post wasn't clear when I mentioned "...(or string.Format(), for that matter)". To be clear, I'm well aware of the string.Format() method. However, in my specific project that I'm working on, string.Format() doesn't help me (it's actually worse than the + concatenator).
Also, I'm inferring that most/all of you are wondering what the motive behind my question is (I suppose I'd feel the same way if I read my question as is).
If you are one of the curious, here's the short of it:
I'm creating a web app running on a Windows CE device. Due to how the web server works, I create the entire web page content (css, js, html, etc) within a string variable. For example, my .cs managed code might have something like this:
string GetPageData()
{
string title = "Hello";
DateTime date = DateTime.Now;
string html = #"
<!DOCTYPE html PUBLIC ...>
<html>
<head>
<title>$title</title>
</head>
<body>
<div>Hello StackO</div>
<div>The date is $date</div>
</body>
</html>
";
}
As you can see, having the ability to specify a variable without the need to concatenate, makes things a bit easier - especially when the content increases in size.

No, unfortunately C# is not PHP.
On the bright side though, C# is not PHP.

Almost, with a small extension method.
static class StringExtensions
{
public static string PHPIt<T>(this string s, T values, string prefix = "$")
{
var sb = new StringBuilder(s);
foreach(var p in typeof(T).GetProperties(BindingFlags.Public | BindingFlags.Instance))
{
sb = sb.Replace(prefix + p.Name, p.GetValue(values, null).ToString());
}
return sb.ToString();
}
}
And now we can write:
string foo = "Bar";
int cool = 2;
var result = "This is a string $foo with $cool variables"
.PHPIt(new {
foo,
cool
});
//result == "This is a string Bar with 2 variables"

No, it doesn't. There are ways around this, but they defeat the purpose. Easiest thing for your example is
Console.WriteLine("The date is {0}", DateTime.Now);

string output = "the date is $d and time is $t";
output = output.Replace("$t", t).Replace("$d", d); //and so on

Based on the great answer of #JesperPalm I found another interesting solution which let's you use a similar syntax like in the normal string.Format method:
public static class StringExtensions
{
public static string Replace<T>(this string text, T values)
{
var sb = new StringBuilder(text);
var properties = typeof(T)
.GetProperties(BindingFlags.Public | BindingFlags.Instance)
.ToArray();
var args = properties
.Select(p => p.GetValue(values, null))
.ToArray();
for (var i = 0; i < properties.Length; i++)
{
var oldValue = string.Format("{{{0}", properties[i].Name);
var newValue = string.Format("{{{0}", i);
sb.Replace(oldValue, newValue);
}
var format = sb.ToString();
return string.Format(format, args);
}
}
This gives you the possibility to add the usual formatting:
var hello = "Good morning";
var world = "Mr. Doe";
var s = "{hello} {world}! It is {Now:HH:mm}."
.Replace(new { hello, world, DateTime.Now });
Console.WriteLine(s); // -> Good morning Mr. Doe! It is 13:54.

The short and simple answer is: No!

string.Format("The date is {0}", DateTime.Now.ToString())

No, But you can create an extension method on the string instance to make the typing shorter.
string s = "The date is {0}".Format(d);

string.Format (and similar formatting functions such as StringBuilder.AppendFormat) are the best way to do this in terms of flexibility, coding practice, and (usually) performance:
string s = string.Format("The date is {0}", d);
You can also specify the display format of your DateTime, as well as inserting more than one object into the string. Check out MSDN's page on the string.Format method.
Certain types also have overloads to their ToString methods which allow you to specify a format string. You could also create an extension method for string that allows you to specify a format and/or parse syntax like this.

How about using the T4 templating engine?
http://visualstudiomagazine.com/articles/2009/05/01/visual-studios-t4-code-generation.aspx

If you are just trying to avoid concatenation of immutable strings, what you're looking for is StringBuilder.
Usage:
string parameterName = "Example";
int parameterValue = 1;
Stringbuilder builder = new StringBuilder();
builder.Append("The output parameter ");
builder.Append(parameterName);
builder.Append("'s value is ");
builder.Append(parameterValue.ToString());
string totalExample = builder.ToString();

Since C# 6.0 you can write string "The title is \{title}" which does exactly what you need.

you can use something like this as mentioned in C# documentation.
string interpolation
string name = "Horace";
int age = 34;
Console.WriteLine($"He asked, \"Is your name {name}?\", but didn't wait for a reply :-{{");
Console.WriteLine($"{name} is {age} year{(age == 1 ? "" : "s")} old.");

Or combined:
Console.WriteLine("The date is {0}", DateTime.Now);
Extra info (in response to BrandonZeider):
Yep, it is kind-a important for people to realize that string conversion is automatically done. Manually adding ToString is broken, e.g.:
string value = null;
Console.WriteLine("The value is '{0}'", value); // OK
Console.WriteLine("The value is '{0}'", value.ToString()); // FAILURE
Also, this becomes a lot less trivial once you realize that the stringification is not equivalent to using .ToString(). You can have format specifiers, and even custom format format providers... It is interesting enough to teach people to leverage String.Format instead of doing it manually.