to understand the Boolean key use in C#

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2.60/5 (5 votes)

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Hi All,
I m trying to execute following code in C# .Net

C#

private void txtRollNumber_KeyDown(object sender, KeyEventArgs e)
{
    label1.Enabled = true;
    bool result = false;
    if (txtRollNumber.Text == false)                    
    {                    
        label1.Text="Please Enter value in Roll Number";
        txtRollNumber.Select();
    }
    else
    {
        txtname.Select();
    }
}

but it shows an error in this line (txtRollNumber.Text == false) it shows that

Operator '==' cannot be applied to operands of type 'string' and 'bool'
Please help what should I do ?

Thanks & Regards
Indrajit

Posted 11-Apr-11 0:18am

IndrajitDasgupat

Updated 11-Apr-11 0:29am

Manfred Rudolf Bihy

v2

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Comments

Kim Togo 11-Apr-11 7:01am

What type of class is "txtRollNumber" ?, TextBox control etc.

4 solutions

Solution 2

You can compare strings with booleans like you may do with apples and computers ( :rolleyes: ).
To fix the issue compare an expression with another of the same type, e.g.

// string to string comparison
if ( txtRollNumberTxt.Text == String.Empty )

or

// bool to bool comparison
if ( String.IsNullOrEmpty(txtRollNumberTxt.Text) == true )

Please note the above code maybe expressed more concisely with

if ( String.IsNullOrEmpty(txtRollNumberTxt.Text) )

Posted 11-Apr-11 0:50am

CPallini

Updated 11-Apr-11 8:07am

v6

Comments

Kim Togo 11-Apr-11 6:58am

IsNullOrEmpty is a static function under String class. "txtRollNumberTxt.Text.IsNullOrEmpty()" is not possible out of the box. Via extension method under .NET, then yes you can make something like this.

CPallini 11-Apr-11 8:05am

You are right, of course. I have (hopefully) fixed my answer (SAKryukov would say the mistake was caused by my C++ imprinting). Thank you.

Kim Togo 11-Apr-11 8:06am

C++ imprinting.. Hehe :-)

Sergey Alexandrovich Kryukov 11-Apr-11 13:14pm

The comparison with Empty is good (with "" is not), but it won't compile: string.Empty is a read-only static property, not a function. Please fix by removing ().
--SA

CPallini 11-Apr-11 14:10pm

Oh my God! My C# is definitely rusty. Thank you and my apologies to the OP.

Sergey Alexandrovich Kryukov 11-Apr-11 14:39pm

You're welcome, and you got my 5 after you put the fix :-)
--SA

Solution 4

SIMPLEST...

if ( txtRollNumberTxt.Text.Trim() == String.Empty )
{
    label1.Text="Please Enter value in Roll Number";
    txtRollNumber.Select();
}
.
.
.

Posted 11-Apr-11 9:33am

web works

Solution 5

go and check the datatype that you are used it

Posted 25-May-11 3:36am

Pritishdeshmukh

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Kim Togo · Accepted Answer · 2011-04-11T01:04:00

Try this.

C#

private void txtRollNumber_KeyDown(object sender, KeyEventArgs e)
{
    label1.Enabled = true;
    bool result = false;
    if (string.IsNullOrWhiteSpace(txtRollNumber.Text))
    {
        label1.Text="Please Enter value in Roll Number";
        txtRollNumber.Select();
    }
    else
    {
        txtname.Select();
    }
}

One more thing.

String.IsNullOrWhiteSpace vs String.IsNullOrEmpty.

The code behind String.IsNullOrWhiteSpace is described like this:

return String.IsNullOrEmpty(value) || value.Trim().Length == 0;