Use string.Contains() with switch()

Question

I'm doing an C# app where I use

if ((message.Contains("test")))
{
   Console.WriteLine("yes");
} else if ((message.Contains("test2"))) {
   Console.WriteLine("yes for test2");
}

There would be any way to change to switch() the if() statements?

Solution 1

You can do the check at first and then use the switch as you like.

For example:

string str = "parameter"; // test1..test2..test3....

if (!message.Contains(str)) return ;

Then

switch(str)
{
  case "test1" : {} break;
  case "test2" : {} break;
  default : {} break;
}

Solution 2

Correct final syntax for [Mr. C]s answer.

With the release of VS2017RC and its C#7 support it works this way:

switch(message)
{
    case string a when a.Contains("test2"): return "no";
    case string b when b.Contains("test"): return "yes";
}

You should take care of the case ordering as the first match will be picked. That's why "test2" is placed prior to test.

Solution 3

Nope, switch statement requires compile time constants. The statement message.Contains("test") can evaluate true or false depending on the message so it is not a constant thus cannot be used as a 'case' for switch statement.

Solution 4

If you just want to use switch/case, you can do something like this, pseudo-code:

    string message = "test of mine";
    string[] keys = new string[] {"test2",  "test"  };

    string sKeyResult = keys.FirstOrDefault<string>(s=>message.Contains(s));

    switch (sKeyResult)
    {
        case "test":
            Console.WriteLine("yes for test");
            break;
        case "test2":
            Console.WriteLine("yes for test2");
            break;
    }

But if the quantity of keys is a big, you can just replace it with dictionary, like this:

static Dictionary<string, string> dict = new Dictionary<string, string>();
static void Main(string[] args)
{
    string message = "test of mine";      

    // this happens only once, during initialization, this is just sample code
    dict.Add("test", "yes");
    dict.Add("test2", "yes2"); 


    string sKeyResult = dict.Keys.FirstOrDefault<string>(s=>message.Contains(s));

    Console.WriteLine(dict[sKeyResult]); //or `TryGetValue`... 
 }

Solution 5

This will work in C# 8 using a switch expresion

var message = "Some test message";

message = message switch
{
    string a when a.Contains("test") => "yes",
    string b when b.Contains("test2") => "yes for test2",
    _ => "nothing to say"
};

For further references https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/switch-expression

View more solutions

Comments

[pmerino]

So I only can use if()? That's pretty messy :(

[Jon Skeet]

@zad0xsis: Do you have a lot of these? If so, you could potentially abstract the idea somewhat...

[pmerino]

yeah I've got a lot of if() :(

[Mr. C]

It's worth noting - this will change with C# 7. Patterns can be used in case clauses. As of this writing, this feature is available in Visual Studio 15 Preview 4. blogs.msdn.microsoft.com/dotnet/2016/08/24/…

[user9993]

No, this does not work. Contains does not exist in the current context.

[Phillip Quinlan]

I removed <string> from string sKeyResult = keys.FirstOrDefault<string>(s=>message.Contains(s)); and still works fine.

[Willy David Jr]

I cannot see your example on the link provided.

[pogosama]

Does not work, you should test it first before sharing your answer. See Lakedaimon's solution for the correct syntax.

[Jeroen Heier]

Hi, welcome to stack overflow. For a useful answer explain why this is an answer to the question.

[Admin]

in C# switch condition is not powerful as KOTLIN so if you want to implement String.contains() in condition state of switch I think we have to separate our condition in default part

[Patrick]

This is pretty close. Mr. C is correct in his answer. "This will work in C# 7" The syntax for this to work is slightly off though... Review Lakedaimon's answer for correct syntax.

[Patrick]

+1 This works perfectly now. And to anyone wondering you can access the value of the case via the "x" variable in this scenario.

[Mark Monforti]

Thanks this worked perfectly for me, I had one small change that might help others that don't just have string message private static string GetRoles(IEnumerable<External.Role> roles) Here was my switch statement. switch (switchStrings.FirstOrDefault<string>(s => roles.Select(t => t.RoleName).Contains(s)))

[NovaDev]

I don't understand this answer.

[Bob Lokerse]

In the scenario of str = "test1,test2"; case "test1" will not work.

[Yumi Koizumi]

Also, you can use any var name-except-the one in the switch statement to evaluate. Then, you can refer to the original variable/expression in the code that follows.

[David Mays]

@Patrick, can you clarify what you mean by the "x" variable? Sorry I'm sure it's obvious.

[David Mays]

I feel so dumb asking about this simple answer, but isn't "break" also mandatory in C# case statements?

[Lakedaimon]

In case statements you can leave the break if you use return.

[GameSalutes]

I find switch expressions a very strange syntax that is hard to remember. Always find myself looking it up if I haven't used it in a few months...

[k3davis]

If you don't need the value of var s (as you don't in your example) you can use var _ to discard it.