String Interpolation with format variable

Question

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.

You have to use 'the old way' you have been using or wrap it into private method. — Andrew B
– Andrew B, Commented Sep 2, 2015 at 18:24

i3arnon · Accepted Answer · 2015-09-02 19:05:53Z

50

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.

edited Sep 2, 2015 at 19:05

answered Sep 2, 2015 at 18:24

i3arnon

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3 Comments

CallMeLaNN Over a year ago

I agreed. I thought logically "{name}" to "{0}" mapping can be done in run-time at the first place but the string.Format parameters should be compiled inline. So I can't simply introduce new {name2} dynamically in variable.

CallMeLaNN Over a year ago

but at least it should accept const string. The compiler then can find-out the "{...}" token and construct the string.Format parameters. Anyway, not a big deal.

Panagiotis Kanavos Over a year ago

@CallMeLaNN this would work only if the const was defined inside the method, so that the compiler could resolve (and validate) the placeholders. Otherwise, how could the compiler decide whether a class-level constant string was valid, if its placeholders existed only inside methods?

svick · Accepted Answer · 2018-01-13 17:44:54Z

35

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);

edited Jan 13, 2018 at 17:44

answered Sep 2, 2015 at 21:08

svick

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Comments

Ian Kemp - SO dead by AI greed · Accepted Answer · 2024-02-28 13:13:00Z

5

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.Height}":

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.

edited Feb 28, 2024 at 13:13

Ian Kemp - SO dead by AI greed

1

answered Sep 2, 2015 at 22:17

Paulo Morgado

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1 Comment

CallMeLaNN Over a year ago

The localization part is "area: " in your example. However +1 for mention about expression.

Michael Schreiber · Accepted Answer · 2019-12-26 01:13:42Z

2

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.

answered Dec 26, 2019 at 1:13

Michael Schreiber

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Tod · Accepted Answer · 2023-01-28 00:28:12Z

1

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)

answered Jan 28, 2023 at 0:28

Tod

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halfer · Accepted Answer · 2018-09-08 22:25:59Z

0

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}";

edited Sep 8, 2018 at 22:25

halfer

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answered Jul 26, 2018 at 18:36

Dimskiy

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Comments

ivan · Accepted Answer · 2015-09-02 18:47:51Z

-6

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

answered Sep 2, 2015 at 18:47

ivan

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