How do I validate textbox in C# to accept certain format in windows application

Just validate it, if you need. What's the problem?

But better idea is just not to allow the user to enter wrong data, unless the rules are too confusing. For the time-relate data, however, it would be much better to use some kind of "DateTimePicker". Unfortunately you did not inform us what UI framework/library you are using. It can be a .NET FCL control, such as System.Windows.Forms.DateTimePicker, or some 3rd-party control. Always, always tag the UI framework/library you are using.

—SA

Updated:

bool IsMillennium = false;
 private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            string CurrentYear = DateTime.Now.Year.ToString();

            if (e.KeyChar == 8)
            {
                e.Handled = false;  // Accept BackSpace
                return;
            }

            if (!char.IsNumber(e.KeyChar))
            {
                e.Handled = true;  // reject any key press but numbers
                return;
            }

            if (textBox1.TextLength >= 4)  // to know which Millennium wuser enters
            {
                if (textBox1.Text[3] == '1')  // [1]950
                {
                    IsMillennium = false; // this mean we at 1950 ~ 1999
                }
                else if (textBox1.Text[3] == CurrentYear[0]) //Part b
                {
                    IsMillennium = true;   //this mean we at 2000 till current year (AKA 2015)
                }
            }

            if (textBox1.TextLength < 13)   // Make sure we at our text limits
            {
                if (textBox1.TextLength < 3) // for first 3 chars we accept Number only
                {
                    if (char.IsNumber(e.KeyChar))
                    {
                        e.Handled = false;
                    }
                    else
                    {
                        e.Handled = true;
                    }
                }
                else
                {
                    // if the text lenght=3 this mean we enter the year part valdiation
                    // year valdiation have 4 partsto valdiate
                    // [1][9][5][0] till CurrentYear[0][1][2][3] variable

                    if (textBox1.TextLength == 3)
                    {
                        //year Part 1
                        // need to accept date between 1950 ~ Current Year
                        //so we check first year char either 1 for [1]950 and  CurrentYear[0] which is first char in our variable
                        // so it must be between 1 and CurrentYear[0] that's why we convert it to Int
                        var yearchar1 = int.Parse(e.KeyChar.ToString());
                        //if ((yearchar1 == 1 || yearchar1 <= int.Parse(CurrentYear[0].ToString())) && yearchar1 != 0)
                        if (yearchar1 <= int.Parse(CurrentYear[0].ToString()) && yearchar1 != 0)
                        {
                            e.Handled = false;
                        }
                        else
                        {
                            e.Handled = true;
                        }
                    }
                    else if (textBox1.TextLength == 4)
                    {
                        //year Part 2
                        //this part is tricky we can't compare it betwwen 9 and 0 casue this will give us all numbers range 0~9 and that's we don't need 
                        //but wee need to break it into 2 parts
                        //Part A: if last char was 1 then we need 9 to follow it for :"1[9]50"
                        //Part B: if last char was CurrentYear[0] then our key need to be smaller than or equal of CurrentYear[1]

                        var yearchar2 = int.Parse(e.KeyChar.ToString());
                        if (textBox1.Text[3] == '1')  // Part A
                        {
                            if (yearchar2 == 9)
                            {
                                e.Handled = false;
                            }
                            else
                            {
                                e.Handled = true;
                            }
                        }
                        else if (textBox1.Text[3] == CurrentYear[0]) //Part B
                        {
                            if (yearchar2 <= int.Parse(CurrentYear[1].ToString()))  //key need to be smaller than or equal of CurrentYear[1]
                            {
                                e.Handled = false;
                            }
                            else
                            {
                                e.Handled = true;
                            }
                        }
                    }
                    else if (textBox1.TextLength == 5)
                    {
                        //year Part 3
                        //like last part we need to know where we are to do our validation right.
                        // thankfully we got our variable IsMillennium which we set it earlier
                        // and we can use it only after  when there is 4 or more chars in our textbox otherwise it's value will be wrong
                        //IsMillennium = false mean we validate 19[5]0 ~ 19[9]9
                        //IsMillennium = true mean we validate 20[0]0 ~ CurrentYear AKA 20[1]5
                        // Part C: betwwen 5 and 9
                        // Part D: key smaller than or equal CurrentYear[2]
                        var yearchar3 = int.Parse(e.KeyChar.ToString());
                        if (!IsMillennium)
                        {
                            if (yearchar3 >= 5 && yearchar3 <= 9) // Part C
                            {
                                e.Handled = false;
                            }
                            else
                            {
                                e.Handled = true;
                            }
                        }
                        else
                        {
                            if (yearchar3 <= int.Parse(CurrentYear[2].ToString()))  // part D
                            {
                                e.Handled = false;
                            }
                            else
                            {
                                e.Handled = true;
                            }
                        }

                    }
                    else if (textBox1.TextLength == 6)
                    {
                        // year part 4
                        // like other we break our problem to smaller problems so it be easyer to solve
                        // Part E: If IsMillennium = false then it must between 0 and 9   ==== 195[0]
                        // Part F: If IsMillennium = true then it must be smaller than or equal CurrentYear[3] ==== 201[5]
                        // another Trick here Millennium will consist of 2 or more parts
                        // 1st part for 2000~2009  and 2nd part for 2010 to current year
                        // Part I: we can break this more to be valide for ever by check if textBox1.Text[5] <  int.Parse(CurrentYear[2].ToString()) then we accept all number ranges
                        // Part II: but if it equal to our current then we accept
                        var yearchar4 = int.Parse(e.KeyChar.ToString());
                        if (!IsMillennium)
                        {
                            if (yearchar4 >= 0 && yearchar4 <= 9) // Part E
                            {
                                e.Handled = false;
                            }
                            else
                            {
                                e.Handled = true;
                            }
                        }
                        else
                        {    // Part F
                            if (int.Parse(textBox1.Text[5].ToString()) < int.Parse(CurrentYear[2].ToString()))// Part I
                            {
                                e.Handled = false;
                            }
                            else
                            {
                                if (yearchar4 <= int.Parse(CurrentYear[3].ToString()))  // Part II
                                {
                                    e.Handled = false;
                                }
                                else
                                {
                                    e.Handled = true;
                                }
                            }

                        }

                    }
                    else if (textBox1.TextLength > 6)
                    {
                        //TO DO : Six digit unique identifier.
                        if (char.IsNumber(e.KeyChar))
                        {
                            e.Handled = false;
                        }
                        else
                        {
                            e.Handled = true;
                        }
                    }
                }
            }
            else
            {
                // reject any key press after it reach max valid lenght
                e.Handled = true;
            }
        }

Edit :
well, if you done validation on key press then why you need validation on leave you already made it hard to type keys and key press event accept specific keys only.
you can validate on leave if the text have the correct length or not in your case something like that

private void textBox1_Leave(object sender, EventArgs e)
        {
            if(textBox1.TextLength != 10)
            {
                textBox1.Focus();
            }
        }

Please Notice that will make it so hard to user because it will prevent him from leave the textbox until he enter the correct format.