C # string.Format optional parameters
I want to use string.Format
with optional parameters
:
public static void Main(string[] args)
{
//Your code goes here
// Console.WriteLine(string.Format("{0} {1}", "a", "b"));
Console.WriteLine(string.Format("{0} {1}", "a"));
}
since the parameter {1}
is optional and has a default value
Can you help me do this?
thank
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It depends on what you mean by "optional parameter".
If you want to automatically replace null
with the default, the easiest way to do it is to use the null coalescing operator inside the arguments:
String.Format("{0} {1}",
"a",
someNullableVariableContainingB ?? "default value");
If you want to reuse the same formatting string for multiple calls to String.Format like
var myFormatString = "{0} {1}";
var string1 = String.Format(myFormatString, "a", "b");
var string2 = String.Format(myFormatString, "a");
then you're out of luck: String.Format will throw an exception if there are too few arguments and there is no way to specify an "optional parameter" inside the format string. You will need to use something other than String.Format, such as a custom method that replaces missing arguments with their assumed defaults.
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This extension method is not limited to a fixed number of parameters. That is, it will work on strings like "{0}"
, but also "{0} {1}"
, "{0} {1} {2}"
and so on. The downside is that you give the optional argument first and then the optional one. It should be the other way around, but unfortunately the nature of the keyword params
prohibits this. The main drawback is that it ignores the number in curly braces (although the solution could be refactored to include that as well).
public static string FormatOpt(this string s, string optional, params string[] param)
{
StringBuilder result = new StringBuilder();
int index = 0;
bool opened = false;
Stack<string> stack = new Stack<string>(param.Reverse());
foreach(var c in s)
{
if (c == '{')
{
opened = true;
index = result.Length;
}
else if (opened && c == '}')
{
opened = false;
var p = stack.Count > 0 ? stack.Pop() : optional;
var lenToRem = result.Length - index;
result.Remove(index, lenToRem);
result.Append(p);
continue;
}
else if (opened && !Char.IsDigit(c))
{
opened = false;
}
result.Append(c);
}
return result.ToString();
}
And the expected results:
string res1 = "result: {0}, {1}, {2}, {3}".FormatOpt("optional", "first param", "second param");
// result: first param, second param, optional, optional
string res2 = "result: {0}, {1}, {2}, {3}".FormatOpt("optional", "first param");
// result: first param, optional, optional, optional
string res3 = "result: {0}, {1}, {2}, {3}".FormatOpt("optional");
// result: optional, optional, optional, optional
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private static Regex curlyBracketRegex = new Regex("\\{(.+?)\\}");
private static string OptionalFormat(string formatString, params object[] args)
{
var numberOfArguments = curlyBracketRegex.Matches(formatString).Count;
var missingArgumentCount = numberOfArguments - args.Length;
if (missingArgumentCount <= 0) //more argument or just enough
return string.Format(formatString, args);
args = args.Concat(Enumerable.Range(0, missingArgumentCount).Select(_ => string.Empty)).ToArray();
return string.Format(formatString, args);
}
This method above works with simple format strings. The regular expression defines curly braces. If the number of matches is greater than the number of arguments passed, a new array will be created concatenating the original array with empty strings.
Example: OptionalFormat ("{0} # {1}", "apple") // apple #
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You can create an extension method:
public static string MyFormat(this string s, int i, params string[] args){
var t = new List<string>(args);
for(var c = t.Count; c < i; ++c)
t.Add(String.Empty); // or other default
return String.Format(s, t.ToArray());
}
and call:
"{0}:{1} - {2},{3},{4}".MyFormat(5, "ping", "fnord");
However, it forces you to supply the arguments in order, so you cannot skip {3} if you want to install {4}. However, you can add:
for(var x = 0; x < args.Count; ++x){
if(args[x] == null) args[x] = String.Empty;
and call:
"{0}:{1} - {2},{3},{4}".MyFormat(5, "ping", null, "pong");
set {0} and {2}, but defaults to {1}, {3} and {4} to String.Empty;
You can go for automatic i detection, but it is much easier than that.
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