Switch case in C# - a constant value is expected

Question

My code is as follows:

public static void Output(IEnumerable dataSource) where T : class
{   
    dataSourceName = (typeof(T).Name);
    sw         


        

Answer 1

This seems to work for me at least when i tried on visual studio 2017.

public static class Words
{
     public const string temp = "What";
     public const string temp2 = "the";
}
var i = "the";

switch (i)
{
  case Words.temp:
    break;
  case Words.temp2:
    break;
}

Answer 2

You can't use a switch statement for this as the case values cannot be evaluated expressions. For this you have to use an an if/else ...

public static void Output<T>(IEnumerable<T> dataSource) where T : class
{   
    dataSourceName = (typeof(T).Name);
    if(string.Compare(dataSourceName, typeof(CustomerDetails).Name.ToString(), true)==0)
    {
        var t = 123;
    }
    else if (/*case 2 conditional*/)
    {
        //blah
    }
    else
    {
        //default case
        Console.WriteLine("Test");
    }
}

I also took the liberty of tidying up your conditional statement. There is no need to cast to string after calling ToString(). This will always return a string anyway. When comparing strings for equality, bare in mind that using the == operator will result in a case sensitive comparison. Better to use string compare = 0 with the last argument to set case sensitive on/off.

Answer 3

See C# switch statement limitations - why?

Basically Switches cannot have evaluated statements in the case statement. They must be statically evaluated.

Answer 4

You can only match to constants in switch statements.

---

Example:

switch (variable1)
{
    case 1: // A hard-coded value
        // Code
        break;
    default:
        // Code
        break;
}

Successful!

---

switch (variable1)
{
    case variable2:
        // Code
        break;
    default:
        // Code
        break;
}

CS0150 A constant value is expected.

Answer 5

Now you can use nameof:

public static void Output<T>(IEnumerable<T> dataSource) where T : class
{
    string dataSourceName = typeof(T).Name;
    switch (dataSourceName)
    {
        case nameof(CustomerDetails):
            var t = 123;
            break;
        default:
            Console.WriteLine("Test");
    }
}

nameof(CustomerDetails) is basically identical to the string literal "CustomerDetails", but with a compile-time check that it refers to some symbol (to prevent a typo).

nameof appeared in C# 6.0, so after this question was asked.

Answer 6

switch is very picky in the sense that the values in the switch must be a compile time constant. and also the value that's being compared must be a primitive (or string now). For this you should use an if statement.

The reason may go back to the way that C handles them in that it creates a jump table (because the values are compile time constants) and it tries to copy the same semantics by not allowing evaluated values in your cases.