How should I calculate the each letter's count in C#

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i dont know how to do it please help with the code thanks

Just giving you the code will not help you to learn something.
HomeWork problems are simplified versions of the kind of problems you will have to solve in real life, their purpose is learning and practicing.

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i want the output as like
c-2
o-2
d-1
e-2
p-1
r-1
j-1
t-1

Giving result in alphabetic order may be easier to program.
You know how to solve the problem by hand, think about it in a mechanical fashion, this is basically your algorithm.

We do not do your HomeWork.
HomeWork is not set to test your skills at begging other people to do your work, it is set to make you think and to help your teacher to check your understanding of the courses you have taken and also the problems you have at applying them.
Any failure of you will help your teacher spot your weaknesses and set remedial actions.
Any failure of you will help you to learn what works and what don't, it is called 'trial and error' learning.
So, give it a try, reread your lessons and start working. If you are stuck on a specific problem, show your code and explain this exact problem, we might help.

As programmer, your job is to create algorithms that solve specific problems and you can't rely on someone else to eternally do it for you, so there is a time where you will have to learn how to. And the sooner, the better.
When you just ask for the solution, it is like trying to learn to drive a car by having someone else training.
Creating an algorithm is basically finding the maths and make necessary adaptation to fit your actual problem.

You should at least Google it

Here are some examples.
Count All Character Occurrences in a String C# - Stack Overflow[^]

https://www.dotnetperls.com/count-letter-frequencies[^]

CP_count_letter | C# Online Compiler | .NET Fiddle[^]

thank you so much for your encouragement finally I got it using of while loop

string inputtext = textBox1.Text;
string result = string.Empty;
while (inputtext .Length > 0)
{
int count = 0;
for (int i = 0; i < inputtext.Length; i++)
{
if (inputtext[0] == inputtext[i])
{
count++;
}
}
result += inputtext[0] + " = " + count + "\n";
inputtext = inputtext.Replace(inputtext[0].ToString(), string.Empty);
}
label1.Text = result;