Operator ‘==’ cannot be applied to operands of type ‘char’ and ‘string’

Question

I’m working on a self directed simple program to practice concepts I’ve learned thus far. My project is related to chess, in this case specifically the board (columns a-h and rows 1-8). The user is asked for the current location of a specific chess piece hopefully entered as a letter for the column followed by a number for the row. To validate this it made sense to me to first check if this value was entered as a string of two characters, otherwise what is entered is already incorrect. I then converted the entered string to lower case characters before comparing it with my list of acceptable array elements.

From searching this site I get the impression that a string stores its characters as an array and using the char property of string you would be able to pull off the first character thus comparing char to char. I have not yet found anything specific enough in my searches to really give me a good understanding of what is happening. This is the closest option I’ve come across which I didn’t feel was applicable to this case. Any insight would be appreciated.

The code that follows produces the following error.

Operator ‘==’ cannot be applied to operands of type ‘char’ and ‘string’

    private char[] gridColumns = new char[] { 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', };

    private void createMoveButton_Click(object sender, RoutedEventArgs e)
    {
        // Assigns text box input to associated private fields
        this.gameId = this.gameIdTextBox.Text;
        this.playerId = this.playerIdTextBox.Text;
        this.gamePiece = this.gamePieceTextBox.Text;
        this.currentLocation = this.currentLocationTextBox.Text;
        this.targetLocation = this.targetLocationTextBox.Text;

        // Current location should only ever be 2 characters, ensure from the start this is true.
        if (currentLocationTextBox.Text.Length == 2)
        {
            // Converts contents of currentLocationTextBox to lower case characters for comparison.
            string cl = currentLocation.ToLowerInvariant();

            // Iterates through my array of possible column choices
            for (int i = 0; i < gridColumns.Length; i++)
            {
                Char.ToLowerInvariant(currentLocationTextBox.Text[0]);
                // Trying to compare the first character of my string to the char element of my array.
                if (cl[0] == gridColumns[i])
                {
                    //TODO
                }
            }
        }
        else
        {
            MessageBox.Show("Check your starting location. It needs to be a lower case character variable (a-h) followed by a number (1-8)");
        }
    }

Answer 1

Unlike C, a string and an array of char are different. A string in C# can be viewed as an array of char but you should consider them different, therefore the '==' comparison isn't appropriate. One easy way to see this is with the following simple expression

   if ("a" == 'a') { /* do something */ } // ERROR!

It looks like it should work but it generates the same error you are seeing, because it is trying to compare the string "a" to the char 'a'. In your example code the Text property of your textbox control is of type string.

The string class has an indexer that allows you to treat a string as an array of char, but it's usually better (simpler) to use one of the many string methods to accomplish your goal. Consider this:

        var gridcolumns = "abcdefgh";
        var gridrows = "12345678";
        var input = "a1"; // column row
        var col = gridcolumns.IndexOf(input[0]); // 0 -7
        var row = gridrows.IndexOf(input[1]); // 0 -7

In the code you gave I don't see a line that would generate the error you provided. The following line serves no purpose

           Char.ToLowerInvariant(currentLocationTextBox.Text[0]);

Because you are not assigning the returned value to a variable, plus 'cl' already contains the lowercasing of that particular value.

This line

            if (cl[0] == gridColumns[i])

should not generate the error because both items are of type char.

Answer 2

Dweeberly's answer when applied... and shortened: Answer: Change the single quotes to double-quotes.

My reasoning: Assume the following code:

string IAmAString;
// set string to anything
IAmAString = "Q";
if (IAmAString == 'Q')
{
 // do something, but never gets here because "Q" is a string, and 'Q' is a char
 // Intellisense gives error on the if statement of 
 // 
 // "Operator '==' cannot be applied to operands of types 'string' and 'char'"
 //
 // because C# is a strongly typed language, and the '==' is not by 
 // default (in VS2012) overloaded to compare these two different types.
 // You are trying to compare a string with something that
 // is not-string, and C# is trying to let you know 
 // that that is not going to work.
}

It is fixed by changing the quotes to double-quotes. C# definitely seems to consider single quotes around a single character to be Char, and not string.

just change the quotes within the if statement:

IAmAString = "Q";
if (IAmAString == "Q")
{
 // does whatever is here within reason; "Q" is a string and "Q" is a string
}

At least this worked for me, and that's my 'quick' (where 'q' != "q") explanation of why.
Getting back to work...