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).
Example
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() {}
}