问题
How can I combine return and switch case statements?
I want something like
return switch(a)
{
case 1:"lalala"
case 2:"blalbla"
case 3:"lolollo"
default:"default"
};
I know about this solution
switch(a)
{
case 1: return "lalala";
case 2: return "blalbla";
case 3: return "lolollo";
default: return "default";
}
But I want to only use the return operator.
回答1:
switch and return can't combine that way, because switch is a statement, not an expression (i.e., it doesn't return a value).
If you really want to use just a single return, you could make a Dictionary to map the switch variable to return values:
var map = new Dictionary<int, string>()
{
{1, "lala"},
{2, "lolo"},
{3, "haha"},
};
string output;
return map.TryGetValue(a, out output) ? output : "default";
回答2:
I believe that this solution is the most straighforward one, and you should definitely use it:
switch(a) {
case 1: return "lalala";
case 2: return "blabla";
case 3: return "lololo";
default: return "default";
}
But, since you asked for one return, you could use this little fluent class:
public class Switch<TElement, TResult> {
TElement _element;
TElement _currentCase;
IDictionary<TElement, TResult> _map = new Dictionary<TElement, TResult>();
public Switch(TElement element) { _element = element; }
public Switch<TElement, TResult> Case(TElement element) {
_currentCase = element;
return this;
}
public Switch<TElement, TResult> Then(TResult result) {
_map.Add(_currentCase, result);
return this;
}
public TResult Default(TResult defaultResult) {
TResult result;
if (_map.TryGetValue(_element, out result)) {
return result;
}
return defaultResult;
}
}
To create code like this:
return new Switch<int, string>(a)
.Case(1).Then("lalala")
.Case(2).Then("blabla")
.Case(3).Then("lololo")
.Default("default");
Unfortunately, the type parameters could not be inferred by the compiler, and it feels a bit clumsy. The Default will trigger the evaluation of the "switch", and must be the last method call in the chain. Note that you always need a default value, since you've turned switch into an expression.
UPDATE: You can solve the type inference problem and drive the user to do the right thing with this code:
public static class Switch {
public static SwitchBuilder<TElement>.CaseBuilder On<TElement>(TElement element) {
return new SwitchBuilder<TElement>(element).Start();
}
public class SwitchBuilder<TElement> {
TElement _element;
TElement _firstCase;
internal SwitchBuilder(TElement element) { _element = element; }
internal CaseBuilder Start() {
return new CaseBuilder() { Switch = this };
}
private ThenBuilder Case(TElement element) {
_firstCase = element;
return new ThenBuilder() { Switch = this };
}
private SwitchBuilder<TElement, TResult>.CaseBuilder Then<TResult>(TResult result) {
return new SwitchBuilder<TElement, TResult>(
_element,
_firstCase,
result).Start();
}
public class CaseBuilder {
internal SwitchBuilder<TElement> Switch { get; set; }
public ThenBuilder Case(TElement element) {
return Switch.Case(element);
}
}
public class ThenBuilder {
internal SwitchBuilder<TElement> Switch { get; set; }
public SwitchBuilder<TElement, TResult>.CaseBuilder Then<TResult>(TResult result) {
return Switch.Then(result);
}
}
}
public class SwitchBuilder<TElement, TResult> {
TElement _element;
TElement _currentCase;
IDictionary<TElement, TResult> _map = new Dictionary<TElement, TResult>();
internal SwitchBuilder(TElement element, TElement firstCase, TResult firstResult) {
_element = element;
_map.Add(firstCase, firstResult);
}
internal CaseBuilder Start() {
return new CaseBuilder() { Switch = this };
}
private ThenBuilder Case(TElement element) {
_currentCase = element;
return new ThenBuilder() { Switch = this };
}
private CaseBuilder Then(TResult result) {
_map.Add(_currentCase, result);
return new CaseBuilder() { Switch = this };
}
private TResult Default(TResult defaultResult) {
TResult result;
if (_map.TryGetValue(_element, out result)) {
return result;
}
return defaultResult;
}
public class CaseBuilder {
internal SwitchBuilder<TElement, TResult> Switch { get; set; }
public ThenBuilder Case(TElement element) {
return Switch.Case(element);
}
public TResult Default(TResult defaultResult) {
return Switch.Default(defaultResult);
}
}
public class ThenBuilder {
internal SwitchBuilder<TElement, TResult> Switch { get; set; }
public CaseBuilder Then(TResult result) {
return Switch.Then(result);
}
}
}
}
The result is this nice, type-safe, fluent interface; where at each step you'll only have the right choice of methods to call (e.g. Then after Case):
return Switch.On(a)
.Case(1).Then("lalala")
.Case(2).Then("blabla")
.Case(3).Then("lololo")
.Default("default");
回答3:
I normally do it this way:
var result = null;
switch(a)
{
case 1:
result = "lalala";
break;
case 2:
result = "blalbla";
break;
case 3:
result = "lolollo";
break;
default:
result = "default";
break;
};
return result;
回答4:
Actually this is possible using switch expressions starting with C# 8.
return a switch
{
1 => "lalala",
2 => "blalbla",
3 => "lolollo",
_ => "default"
};
Switch Expressions
There are several syntax improvements here:
- The variable comes before the switch keyword. The different order makes it visually easy to distinguish the switch expression from the switch statement.
- The case and : elements are replaced with =>. It's more concise and intuitive.
- The default case is replaced with a _ discard.
- The bodies are expressions, not statements.
For more information and examples check the Microsoft's C# 8 Whats New.
回答5:
switch(a)
{
case 1: return "lalala";
case 2: return "blalbla";
case 3: return "lolollo";
default: return "default";
}
回答6:
With the new C# 8, you can combine both return and switch. The new switch is so cute.
public static RGBColor FromRainbow(Rainbow colorBand) =>
colorBand switch
{
Rainbow.Red => new RGBColor(0xFF, 0x00, 0x00),
Rainbow.Orange => new RGBColor(0xFF, 0x7F, 0x00),
Rainbow.Yellow => new RGBColor(0xFF, 0xFF, 0x00),
Rainbow.Green => new RGBColor(0x00, 0xFF, 0x00),
Rainbow.Blue => new RGBColor(0x00, 0x00, 0xFF),
Rainbow.Indigo => new RGBColor(0x4B, 0x00, 0x82),
Rainbow.Violet => new RGBColor(0x94, 0x00, 0xD3),
_ => throw new ArgumentException(message: "invalid enum value", paramName: nameof(colorBand))
};
The equivalent old switch is as below.
public static RGBColor FromRainbowClassic(Rainbow colorBand)
{
switch (colorBand)
{
case Rainbow.Red:
return new RGBColor(0xFF, 0x00, 0x00);
case Rainbow.Orange:
return new RGBColor(0xFF, 0x7F, 0x00);
case Rainbow.Yellow:
return new RGBColor(0xFF, 0xFF, 0x00);
case Rainbow.Green:
return new RGBColor(0x00, 0xFF, 0x00);
case Rainbow.Blue:
return new RGBColor(0x00, 0x00, 0xFF);
case Rainbow.Indigo:
return new RGBColor(0x4B, 0x00, 0x82);
case Rainbow.Violet:
return new RGBColor(0x94, 0x00, 0xD3);
default:
throw new ArgumentException(message: "invalid enum value", paramName: nameof(colorBand));
};
}
You can read about this feature here.
回答7:
This is the closest I can think of:
return a==1 ? "lalala"
: a==2 ? "blalbla"
: a==3 ? "lolollo"
: "default";
回答8:
My mapping solution looks like Jordão's solution but it is more flexible and shorter.
return a
.Map(1,"lalala")
.Map(2,"blabla")
.Map(3,"lololo")
.Else(string.Empty);
Both arguments can also be a function:
return a
.Map(x => x <= 0, "lalala")
.Map(2, "blabla")
.Map(x => x >= 3, x => "lololo" + x.ToString()); // lololo3 etc.
回答9:
I've created a Nuget package (FluentSwitch) that should do what you want. So you can do the following:
var result = myValue.Switch()
.When(1, "lalala")
.When(2, "blalbla")
.When(3, "lolollo")
.Else("default")
.Value();
回答10:
If you want switch to return value, you can use delegate:
int a = 2;
string result = new Func<string>(delegate ()
{
switch (a)
{
case 1: return "lalala";
case 2: return "blalbla";
case 3: return "lolollo";
default: return "default";
}
})();
Or:
int a = 2;
string result = new Func<int,string>(delegate (int i)
{
switch (i)
{
case 1: return "lalala";
case 2: return "blalbla";
case 3: return "lolollo";
default: return "default";
}
})(a);
Or just use lambda:
int a = 2;
string result = new Func<int,string>((int i) =>
{
switch (i)
{
case 1: return "lalala";
case 2: return "blalbla";
case 3: return "lolollo";
default: return "default";
}
})(a);
回答11:
public String doStaff(int a) {
switch(a)
{
case 1: return "lalala"
case 2: return "blalbla"
case 3: return "lolollo"
default: return "default"
};
}
回答12:
We can have one use case where we may need to return the value from condition written inside the switch; let's say:
public void SomeMethod(SomeType enumType)
{
switch (enumType)
{
case a:
if (condition)
{
if (condition1 && condition2)
{
return true;
}
}
return false;
//break; break is actually not be required here if return used before break
case b:
if (condition)
{
if (condition3 && condition4)
{
return true;
}
}
return false;
// break;
default:
return false;
//break;
}
Public enum SomeType
{
a,
b,
c,
d
}
来源：https://stackoverflow.com/questions/3334138/combine-return-and-switch