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Compiler Error CS0019
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Compiler Error CS0019

Operator 'operator' cannot be applied to operands of type 'type' and 'type'

A binary operator is applied to data types that do not support it. For example, you cannot use the || operator on strings, you cannot use + , - , < , or > operators on bool variables, and you cannot use the == operator with a struct type unless the type explicitly overloads that operator.

If you encounter this error with a class type, it is because the class does not overload the operator. For more information, see Overloadable Operators (C# Programming Guide).

In the following example, CS0019 is generated in two places because bool in C# is not convertible to int. CS0019 also is generated when the subtraction operator is applied to a string. The addition operator (+) can be used with string operands because that operator is overloaded by the String class to perform string concatenation.

static void Main()
    bool result = true;
    if (result > 0) //CS0019
        // Do something.

    int i = 1;
    // You cannot compare an integer and a boolean value.
    if (i == true) //CS0019
        //Do something...

    // The following use of == causes no error. It is the comparison of
    // an integer and a boolean value that causes the error in the 
    // previous if statement.
    if (result == true)
        //Do something...

    string s = "Just try to subtract me.";
    float f = 100 - s; // CS0019

In the following example, conditional logic must be specified outside the ConditionalAttribute. You can pass only one predefined symbol to the ConditionalAttribute.

The following sample generates CS0019.

// CS0019_a.cs
// compile with: /target:library
using System.Diagnostics;
public class MyClass
   [ConditionalAttribute("DEBUG" || "TRACE")]   // CS0019
   public void TestMethod() {}

   // OK
   [ConditionalAttribute("DEBUG"), ConditionalAttribute("TRACE")]
   public void TestMethod2() {}
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