csharp coding doubt

what is the meaning of {0}
{}: This tells it is one of those place where some string will be replaced when the line of code will be executed.
0: This tells that the string defined at 0th position in the order will be picked for replacement. So, it's the index to be picked for replacement.

For clear example:

Console.WriteLine("Summed Values = {0}, Got it{1}", sum, mark); 

0 -> sum
1 -> mark

This kind of representation helps you to show something at runtime with ease. If you are not sure of certain values at runtime and want to collate few in a single output/statement, this syntax helps.

All the others have already provided the answer.
I just want to add that you can use this placeholder for formatting a string as well.

E.g. String.Format("The number {0} in Hexadecimal is {0:X}", 432);

Here is a set of formatters

Control	Description
C	Displays number prefixed with the currency simple appropriate to the current locale
D	Displays number in decimal form with optional padding
E	Displays number in scientific form with optional value for fractional part
E	Displays the number including the specified number of decimal digits
N	Converts a number to a human friendly format by inserting commas and rounding to the nearest 100th
X	Converts a number to hexadecimal
0:0...	Adds zeros to pad argument
0:0#...	Adds spaces to pad argument
%	Multiplies the argument by 100 and appends a percentage sign

(Table borrowed from http://www.techotopia.com/index.php/Formatting_Strings_in_C_Sharp[^]).

The answers given are very good.

I want to add that the Field width and Field alignment can be given using the Format item {0},{1} etc. as shown below:

Console.WriteLine("| {0,-20} | {1,20} |","Left Aligned","Right Aligned");
Console.WriteLine("| {0,-20} | {1,20:N2} |","Item1",20);
Console.WriteLine("| {0,-20} | {1,20:N2} |","Item2",50.545);
Console.WriteLine("| {0,-20} | {1,20:N2} |","Item3",30.396);

//| Left Aligned         |        Right Aligned |
//| Item1                |                20.00 |
//| Item2                |                50.55 |
//| Item3                |                30.40 |

-20 indicates left alignment with 20 characters width and 20 indicates right alignment with 20 characters width.

Of course to see the correct alignment and width a Monospaced font, like Courier New, explained here Monospaced font[^] is required.

Hi,

It's a place holder to hold the value in the memory.

Console.WriteLine("Summed Values = {0}, {1}, {2}", sum, sum1, sum2);

You can also use multiple placeholder. It creates a array in the memory where values can be stored. Instead of using this you can also use concatenation operator like this:

Console.WriteLine("Summed Values = "+ sum);

To print more than one values you can also use this like:

Console.WriteLine("Summed Values = '"+ sum+"', '"+sum1+"'");

-AK

all above are correct. i would like to give explanation in different way.

syntax of writeline is :
Console.WriteLine("string to be printed");
but if u have any value of a variable to be printed then u need to provide it to WriteLine somehow.
for that v have another syntax:
Console.WriteLine("<string to="" be="" printed="">",<variables whose="" value="" is="" to="" be="" printed="">);

now the list contains variable names each separated by comma(,)
WriteLine identifies each variable in the list by its position i.e.
first variable in the list has index 0
second variable in the list has index 1

and so on.

but to print the value of the variable, it need to b included in the "string to be printed" section of WriteLine .
we provide this by using the index and to notify the method that it is index and not a character to be printed we provide the index within "{}"(braces)

so to print first variable in the list v provide index 0[i.e. we write {0} within "string to be printed"]
the value of variable will b printed in position where {} is positioned.
eg:-

//a,b are variables
Console.WriteLine("{0} + {1} = {2}", a, b, (a+b));
// <value of a> + <value of b> = <value of (a+b)>