如何使 C # Switch 语句使用 IgnoreCase

如果我有一个 switch-case 语句，其中开关中的对象是字符串，是否可以进行 IgnoreCase 比较？

例如:

string s = "house";
switch (s)
{
case "houSe": s = "window";
}

s会得到值“窗口”吗？我如何覆盖 switch-case 语句，以便它使用 ignoreCase 来比较字符串？

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A simpler approach is just lowercasing your string before it goes into the switch statement, and have the cases lower.

Actually, upper is a bit better from a pure extreme nanosecond performance standpoint, but less natural to look at.

E.g.:

string s = "house";
switch (s.ToLower()) {
case "house":
s = "window";
break;
}
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最佳答案

As you seem to be aware, lowercasing two strings and comparing them is not the same as doing an ignore-case comparison. There are lots of reasons for this. For example, the Unicode standard allows text with diacritics to be encoded multiple ways. Some characters includes both the base character and the diacritic in a single code point. These characters may also be represented as the base character followed by a combining diacritic character. These two representations are equal for all purposes, and the culture-aware string comparisons in the .NET Framework will correctly identify them as equal, with either the CurrentCulture or the InvariantCulture (with or without IgnoreCase). An ordinal comparison, on the other hand, will incorrectly regard them as unequal.

Unfortunately, switch doesn't do anything but an ordinal comparison. An ordinal comparison is fine for certain kinds of applications, like parsing an ASCII file with rigidly defined codes, but ordinal string comparison is wrong for most other uses.

What I have done in the past to get the correct behavior is just mock up my own switch statement. There are lots of ways to do this. One way would be to create a List<T> of pairs of case strings and delegates. The list can be searched using the proper string comparison. When the match is found then the associated delegate may be invoked.

Another option is to do the obvious chain of if statements. This usually turns out to be not as bad as it sounds, since the structure is very regular.

The great thing about this is that there isn't really any performance penalty in mocking up your own switch functionality when comparing against strings. The system isn't going to make a O(1) jump table the way it can with integers, so it's going to be comparing each string one at a time anyway.

If there are many cases to be compared, and performance is an issue, then the List<T> option described above could be replaced with a sorted dictionary or hash table. Then the performance may potentially match or exceed the switch statement option.

Here is an example of the list of delegates:

delegate void CustomSwitchDestination();
List<KeyValuePair<string, CustomSwitchDestination>> customSwitchList;
CustomSwitchDestination defaultSwitchDestination = new CustomSwitchDestination(NoMatchFound);
void CustomSwitch(string value)
{
foreach (var switchOption in customSwitchList)
if (switchOption.Key.Equals(value, StringComparison.InvariantCultureIgnoreCase))
{
switchOption.Value.Invoke();
return;
}
defaultSwitchDestination.Invoke();
}

Of course, you will probably want to add some standard parameters and possibly a return type to the CustomSwitchDestination delegate. And you'll want to make better names!

If the behavior of each of your cases is not amenable to delegate invocation in this manner, such as if differnt parameters are necessary, then you’re stuck with chained if statments. I’ve also done this a few times.

    if (s.Equals("house", StringComparison.InvariantCultureIgnoreCase))
{
s = "window";
}
else if (s.Equals("business", StringComparison.InvariantCultureIgnoreCase))
{
s = "really big window";
}
else if (s.Equals("school", StringComparison.InvariantCultureIgnoreCase))
{
s = "broken window";
}
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In some cases it might be a good idea to use an enum. So first parse the enum (with ignoreCase flag true) and than have a switch on the enum.

SampleEnum Result;
bool Success = SampleEnum.TryParse(inputText, true, out Result);
if(!Success){
//value was not in the enum values
}else{
switch (Result) {
case SampleEnum.Value1:
break;
case SampleEnum.Value2:
break;
default:
//do default behaviour
break;
}
}
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One possible way would be to use an ignore case dictionary with an action delegate.

string s = null;
var dic = new Dictionary<string, Action>(StringComparer.CurrentCultureIgnoreCase)
{
{"house",  () => s = "window"},
{"house2", () => s = "window2"}
};


dic["HouSe"]();

// Note that the call doesn't return text, but only populates local variable s.
// If you want to return the actual text, replace Action to Func<string> and values in dictionary to something like () => "window2"

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I hope this helps try to convert the whole string into particular case either lower case or Upper case and use the Lowercase string for comparison:

public string ConvertMeasurements(string unitType, string value)
{
switch (unitType.ToLower())
{
case "mmol/l": return (Double.Parse(value) * 0.0555).ToString();
case "mg/dl": return (double.Parse(value) * 18.0182).ToString();
}
}
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Sorry for this new post to an old question, but there is a new option for solving this problem using C# 7 (VS 2017).

C# 7 now offers "pattern matching", and it can be used to address this issue thusly:

string houseName = "house";  // value to be tested, ignoring case
string windowName;   // switch block will set value here


switch (true)
{
case bool b when houseName.Equals("MyHouse", StringComparison.InvariantCultureIgnoreCase):
windowName = "MyWindow";
break;
case bool b when houseName.Equals("YourHouse", StringComparison.InvariantCultureIgnoreCase):
windowName = "YourWindow";
break;
case bool b when houseName.Equals("House", StringComparison.InvariantCultureIgnoreCase):
windowName = "Window";
break;
default:
windowName = null;
break;
}

This solution also deals with the issue mentioned in the answer by @Jeffrey L Whitledge that case-insensitive comparison of strings is not the same as comparing two lower-cased strings.

By the way, there was an interesting article in February 2017 in Visual Studio Magazine describing pattern matching and how it can be used in case blocks. Please have a look: Pattern Matching in C# 7.0 Case Blocks

EDIT

In light of @LewisM's answer, it's important to point out that the switch statement has some new, interesting behavior. That is that if your case statement contains a variable declaration, then the value specified in the switch part is copied into the variable declared in the case. In the following example, the value true is copied into the local variable b. Further to that, the variable b is unused, and exists only so that the when clause to the case statement can exist:

switch(true)
{
case bool b when houseName.Equals("X", StringComparison.InvariantCultureIgnoreCase):
windowName = "X-Window";):
break;
}

As @LewisM points out, this can be used to benefit - that benefit being that the thing being compared is actually in the switch statement, as it is with the classical use of the switch statement. Also, the temporary values declared in the case statement can prevent unwanted or inadvertent changes to the original value:

switch(houseName)
{
case string hn when hn.Equals("X", StringComparison.InvariantCultureIgnoreCase):
windowName = "X-Window";
break;
}
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An extension to the answer by @STLDeveloperA. A new way to do statement evaluation without multiple if statements as of C# 7 is using the pattern matching switch statement, similar to the way @STLDeveloper though this way is switching on the variable being switched

string houseName = "house";  // value to be tested
string s;
switch (houseName)
{
case var name when string.Equals(name, "Bungalow", StringComparison.InvariantCultureIgnoreCase):
s = "Single glazed";
break;
            

case var name when string.Equals(name, "Church", StringComparison.InvariantCultureIgnoreCase):
s = "Stained glass";
break;
...
default:
s = "No windows (cold or dark)";
break;
}

The visual studio magazine has a nice article on pattern matching case blocks that might be worth a look.

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Here's a solution that wraps @Magnus 's solution in a class:

public class SwitchCaseIndependent : IEnumerable<KeyValuePair<string, Action>>
{
private readonly Dictionary<string, Action> _cases = new Dictionary<string, Action>(StringComparer.OrdinalIgnoreCase);


public void Add(string theCase, Action theResult)
{
_cases.Add(theCase, theResult);
}


public Action this[string whichCase]
{
get
{
if (!_cases.ContainsKey(whichCase))
{
throw new ArgumentException($"Error in SwitchCaseIndependent, \"{whichCase}\" is not a valid option");
}
//otherwise
return _cases[whichCase];
}
}


public IEnumerator<KeyValuePair<string, Action>> GetEnumerator()
{
return _cases.GetEnumerator();
}


IEnumerator IEnumerable.GetEnumerator()
{
return _cases.GetEnumerator();
}
}

Here's an example of using it in a simple Windows Form's app:

   var mySwitch = new SwitchCaseIndependent
{
{"hello", () => MessageBox.Show("hello")},
{"Goodbye", () => MessageBox.Show("Goodbye")},
{"SoLong", () => MessageBox.Show("SoLong")},
};
mySwitch["HELLO"]();

If you use lambdas (like the example), you get closures which will capture your local variables (pretty close to the feeling you get from a switch statement).

Since it uses a Dictionary under the covers, it gets O(1) behavior and doesn't rely on walking through the list of strings. Of course, you need to construct that dictionary, and that probably costs more. If you want to reuse the Switch behavior over and over, you can create and initialize the the SwitchCaseIndependent object once and then use it as many times as you want.

It would probably make sense to add a simple bool ContainsCase(string aCase) method that simply calls the dictionary's ContainsKey method.

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It should be sufficient to do this:

string s = "houSe";
switch (s.ToLowerInvariant())
{
case "house": s = "window";
break;
}

The switch comparison is thereby culture invariant. As far as I can see this should achieve the same result as the C#7 Pattern-Matching solutions, but more succinctly.

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I would say that with switch expressions (added in C# 8.0), discard patterns and local functions the approaches suggested by @STLDev and @LewisM can be rewritten in even more clean/shorter way:

string houseName = "house";  // value to be tested
// local method to compare, I prefer to put them at the bottom of the invoking method:
bool Compare(string right) => string.Equals(houseName, right, StringComparison.InvariantCultureIgnoreCase);
var s = houseName switch
{
_ when Compare("Bungalow") => "Single glazed",
_ when Compare("Church") => "Stained glass",
//  ...
_ => "No windows (cold or dark)" // default value
};
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Using the Case Insensitive Comparison: Comparing strings while ignoring case.

switch (caseSwitch)
{
case string s when s.Equals("someValue", StringComparison.InvariantCultureIgnoreCase):
// ...
break;
}

for more detail Visit this link: Switch Case When In C# Statement And Expression


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