C# convert int to string with padding zeros?

i.ToString().PadLeft(4, '0') - okay, but doesn't work for negative numbers
i.ToString("0000"); - explicit form
i.ToString("D4"); - short form format specifier
$"{i:0000}"; - string interpolation (C# 6.0+)

i.ToString("D4");

See MSDN on format specifiers.

Here's a good example:

int number = 1;
//D4 = pad with 0000
string outputValue = String.Format("{0:D4}", number);
Console.WriteLine(outputValue);//Prints 0001
//OR
outputValue = number.ToString().PadLeft(4, '0');
Console.WriteLine(outputValue);//Prints 0001 as well

You can use:

int x = 1;
x.ToString("0000");

C# 6.0 style string interpolation

int i = 1;
var str1 = $"{i:D4}";
var str2 = $"{i:0000}";

i.ToString("0000");

Easy peasy

int i = 1;
i.ToString("0###")

Simply

int i=123;
string paddedI = i.ToString("D4");

.NET has an easy function to do that in the String class. Just use:

.ToString().PadLeft(4, '0')  // that will fill your number with 0 on the left, up to 4 length

int i = 1; 
i.toString().PadLeft(4,'0')  // will return "0001"  

int p = 3; // fixed length padding
int n = 55; // number to test

string t = n.ToString("D" + p); // magic     

Console.WriteLine("Hello, world! >> {0}", t);

// outputs: 
// Hello, world! >> 055

public static string ToLeadZeros(this int strNum, int num)
{
    var str = strNum.ToString();
    return str.PadLeft(str.Length + num, '0');
}

// var i = 1;
// string num = i.ToLeadZeros(5);

Most of the given answers are slow or very slow or don't work for negative numbers.

Try this one:

}
    //
    //
    ///<summary>Format a value with a fixed number of digits.</summary>
    public static string Pad( this long v, int digits ) {
        int negative = 0;
        if ( v < 0 ) {
            negative = 1;
            v = Math.Abs( v );
        }
        var source = v.ToString();
        var length = source.Length;
        int max = length;
        if ( max < digits ) {
            max = digits;
        }
        max += negative;
        var ca = new char[ max ];
        for ( int i = 0; i < max; i++ ) {
            ca[ i ] = '0';
        }
        while ( length > 0 ) {
            ca[ --max ] = source[ --length ];
        }
        if ( 0 != negative ) ca[ 0 ] = '-';
        return new string( ca );
    }

string hello = "Hello C# Corner.";

string helloHash = hello.PadLeft(5, '#');  

Console.WriteLine(helloHash); 

Output :-

#####Hello C# Corner.

Here I want to pad my number with 4 digit. For instance, if it is 1 then it should show as 0001, if it 11 it should show as 0011.

Below is the code that accomplishes this:

reciptno=1; // Pass only integer.

string formatted = string.Format("{0:0000}", reciptno);

TxtRecNo.Text = formatted; // Output=0001

I implemented this code to generate money receipt number for a PDF file.

To pad int i to match the string length of int x, when both can be negative:

i.ToString().PadLeft((int)Math.Log10(Math.Abs(x < 0 ? x * 10 : x)) + 1, '0')