if语句和“？/：”运算符之间的区别

Question

I have this piece of code here. In case ServiceType.Register: I have what appears to be two equivalent statements one using a regular if statement and one using the ternary operator ?/: . For the if statement VS reports no error. However with this line:

IsXML == true ? PopulateRegister(ParseType.Xml) : PopulateRegister(ParseType.Str);

VS is erroring out saying:

Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement

Anyone know why regular if statement has no error but if you use "?/:" (one liner) throws an error? Pic is also attached.

switch (this.ServiceType)
{
    case SerivceType.Login:
        PopulateLogin();
        break;
    case SerivceType.Register:

        if (IsXML == true)
            PopulateRegister(ParseType.Xml);
        else
            PopulateRegister(ParseType.Str);

        IsXML == true ? PopulateRegister(ParseType.Xml) : PopulateRegister(ParseType.Str);

        break;
    case SerivceType.Verify:
        PopulateVerify();
        break;
}

Answer 1

The ?: operator is used for conditional assignments, not operations.

The statement:

IsXML == true ? PopulateRegister(ParseType.Xml) : PopulateRegister(ParseType.Str);

would be valid if the method PopulateRegister returns a value, and if you assigned that value to something. For example, this would be valid:

string result = (someCondition) ? "condition is true" : "condition is false";

You might want to use the conditional this way instead (note that we are using the ParseType enum value as a return type of the condition, and it is acting as the parameter to your method):

PopulateRegister((IsXML) ? ParseType.Xml : ParseType.Str);

Note that the above is possible, but can produce hard to understand/debug/maintain code and would generally not be thought of as a best practice.

Answer 2

A conditional operator is a shortcut to assign a value to a variable. But your method PopulateRegister does not return a value( void ). So use an if-else instead.

Answer 3

The reason for the error is that if..then..else is a statement, meaning it is supposed NOT to return a value, think of it as if it was a void function of sorts (not really so, but i hope you get the idea) while the ? : ? : operator is an expression, meaning it returns a value and since the functions you are using are void there is nothing to return. Hope this helps

Answer 4

IsXML == true ? PopulateRegister(ParseType.Xml) : PopulateRegister(ParseType.Str);

You are not assigning the result to anything.

The conditional operator (?:) returns one of two values depending on the value of a Boolean expression. Following is the syntax for the conditional operator. [Emphasis mine]

https://msdn.microsoft.com/en-us/library/ty67wk28.aspx

There is nothing to return as your functions are of return type void.

Answer 5

if is a statement and ?: is a ternary operator , to be used within an expression . Its operands must be expressions yielding a value. Your method PopulateRegister doesn't return a value, so it won't work.

You cannot provide an arbitrary expression in place of a statement, so what you thought would be a shortcut won't work. The following is not a valid statement:

// this is an expression, not a statement
IsXML == true ? PopulateRegister(ParseType.Xml) : PopulateRegister(ParseType.Str);

This would be valid:

// this is a valid statement
PopulateRegister(IsXML ? ParseType.Xml : ParseType.Str);

---

Side notes:

• Don't use IsXML == true — IsXML is a boolean expression itself, so an equality comparison to true is completely redundant and just clutters your code.
• The error you are observing is compile-time. Hence it is not thrown by the code itself, it's reported by the compiler.

Answer 6

Think of the block: (b) ? (c) : (d) (b) ? (c) : (d) as a one stand-alone object value . Such that b is boolean. c and d are inherited from object .