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Correct use of logical operator in C#

I have the following If block that runs upon pressing a command button on a form object. This should simply check to see if any of the four mentioned text boxes are empty and if so, display a message box then exit that procedure so that the user can correct the fields and continue.
Here is the relevant code:
if (string.IsNullOrWhiteSpace(txtName.ToString()) ||
string.IsNullOrWhiteSpace(txtID.ToString()) ||
string.IsNullOrWhiteSpace(txtSalary.ToString()) ||
string.IsNullOrWhiteSpace(txtERR.ToString()))
{
MessageBox.Show("One or more text fields are empty or hold invalid data, please correct this to continue","Data Error",MessageBoxButtons.OK);
return;
}
I've left all the text fields blank, and even tried putting white space characters in but the conditional code isn't being executed. As the code isn't executing I'm assuming there is something wrong with my if statement, perhaps I'm not using the 'or' operator || correctly? Any help appreciated.

If you are checking textboxes you need to get the text from the textbox.
if (string.IsNullOrWhiteSpace(txtName.Text) || ...
As a little bonus you could also write it like this:
if(new [] {txtName, txtID, txtSalary, txtERR}
.Any(tb => string.IsNullOrWhiteSpace(tb.Text)))
{
MessageBox.Show("One or more text fields are empty or hold invalid data, please correct this to continue","Data Error",MessageBoxButtons.OK);
return;
}

You should use Text property of TextBox. ToString method returns string "System.Windows.Forms.TextBoxBase". This string is obviously never empty or null.
if (string.IsNullOrWhiteSpace(txtName.Text) ||
string.IsNullOrWhiteSpace(txtID.Text) ||
string.IsNullOrWhiteSpace(txtSalary.Text) ||
string.IsNullOrWhiteSpace(txtERR.Text))
{
MessageBox.Show("One or more text fields are empty or hold invalid data, please correct this to continue","Data Error",MessageBoxButtons.OK);
return;
}

If txtName, txtID etc. name of controls then you need to refer to .Text property. Try something like snippet below:
if (string.IsNullOrWhiteSpace(txtName.Text) ||
string.IsNullOrWhiteSpace(txtID.Text) ||
string.IsNullOrWhiteSpace(txtSalary.Text) ||
string.IsNullOrWhiteSpace(txtERR.Text))
{
MessageBox.Show("One or more text fields are empty or hold invalid data, please correct this to continue","Data Error",MessageBoxButtons.OK);
return;
}

TextBox.ToString() will return the type of the TextBox - thus this will never be NullOrWhiteSpace. What you want is to check the contents of the Text property like so:
if (string.IsNullOrWhiteSpace(txtName.Text ||
string.IsNullOrWhiteSpace(txtID.Text) ||
string.IsNullOrWhiteSpace(txtSalary.Text) ||
string.IsNullOrWhiteSpace(txtERR.Text))
{
MessageBox.Show("One or more text fields are empty or hold invalid data, please correct this to continue","Data Error",MessageBoxButtons.OK);
return;
}

I don't use IsNullOrWhiteSpace for that kind of test, instead i prefer to use IsNullOrEmpty
try this:
if (string.IsNullOrEmpty(txtName.Text)||...)
{...
or maybe txtName is returning the TEXT object...try this then
if (string.IsNullOrEmpty(txtName.Text.toString())||...)
{...

On inserting to a database with predefined value C#

OK I have a text box with a set value, I only want this set value sent if the text box is not filled in on completion of form.
code
dontaion_euro.Text = "99.00";
problem is when I click send it only sends the data 99.00 I only want this sent if blank.
how could I achieve this?

Step1 : you need to Trim the Textbox value to remove the white spaces.
Step2 : you can compare the Textbox value with Empty String to check whether Textbox value is Empty or not
Try this:
if(donation_euro.Text.Trim().Equals(""))
{
donation_euro.Text="99.00";
}

If I understand correctly, you can use String.IsNullOrEmpty method like;
Indicates whether the specified string is null or an Empty string.
if(string.IsNullOrEmpty(dontaion_euro.Text))
{
dontaion_euro.Text = "99.00";
}
else
{
//Your textbox is not null or empty
}

textbox Insert and remove function in vb.net or c#

How to update the text from the TextBox control?
Consider a TextBox that already contains the string "Wel"
To insert text in the TextBox, I use:
TextBox1.Text = TextBox1.Text.Insert(3, "come")
And to remove characters from the TextBox:
TextBox1.Text = TextBox1.Text.Remove(3, 4)
But I need to be able to do this:
TextBox1.Text.Insert(3, "come");
TextBox1.Text.Remove(3, 4);
However, this code doesn't update the TextBox.
It this possible?
Can this be accomplished via the append method?

Text property of TextBox is of type string which is immutable it's not possible to change the existing string. Insert() or Remove() returns a new instance of string with the modification and you will have to assign this new instance back to TextBox's Text property.
There is TextBox.AppendText() that you might be interested in. It appends text to the end of the string but you cannot do anything like Insert() or Remove() with it though.
EDIT:
for your keypress, you could do something like this
char charToReplace = (char) (e.KeyChar + 1); // substitute replacement char
textBox1.SelectedText = new string(charToReplace,1);
e.Handled = true;

'string' is a immutable type, so each time string value changes new memory is allocated. So if you want to insert or remove text from TextBox, you have to assign it back to TextBox.Text property. However, if you just want to append text to the TextBox.Text, you can do
textBox.AppendText("Hello");

Decimal digit problem

On Asp.Net, in my textbox, i do not want to enter after zero three numbers.
For example if you enter textbox 0.222 it should be 0.22. After 0.22 you should not enter anything. How can i make this?
Thanks

Use a Masked TextBox.

I think it is better to use an Expression instead of Masked Textbox.
With a simple RegularExpressionValidator you are able to do this, just drag one in your worksheet and then set the TargetControl of it to your Textbox, then set its value to ^\ddd.

Using javascript add an attribute onkeyup='formatNumber(this)' on the tag.
function formatNumber(textBox) {
var current = textBox.value;
var output = "";
//find '000' in current
//delete what you want
textBox.value = output;
}

WPF DataBinding: Nullable Int still gets a validation error?

I have a textbox databound to a nullable int through code. If I erase the data from the textbox it gives me a validation error (red border around it).
Here is my binding code:
ZipBinding = new Binding("Zip");
ZipBinding.Source = Address;
zipTextBox.SetBinding(TextBox.TextProperty, ZipBinding);
public Int32? Zip { get { ... } set { ... } }
It's clearly marked as a Nullable so why does WPF wanna give me a validation issue when I clear the textbox?

Validation is failing because it can't convert the empty string to a nullable integer. Set TargetNullValue to string.empty on the Binding and it will convert the empty string to null, which will be valid.

An empty TextBox != null.
You may have to tweak the ValidationRule to accommodate empty strings as entries. Or, you could create a converter to take empty strings and convert them to null.