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Compiler Error CS0019

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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.

You can overload an operator to make it support operands of certain types. For more information, see Operator overloading.

Example 1

In the following example, CS0019 is generated in three places because bool in C# is not convertible to int. CS0019 is also 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...
    }

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

Example 2

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() {}
}

See also