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How do I add a reference from a for loop to the string format in the WriteLine function?

I have looked about everywhere (exaggeration) before I started asking the question and have come up with nothing, I think the question may be confusing but I am trying to pass the variable i to the parameters in the WriteLine(..) method string, here is an example;
Expression<Func<int, int, int>> expression = (a, b) => a + b;
for(int i = 0; i < expression.Parameters.Count; i++)
{
Console.WriteLine("Expression param[{i+1}]: {i}", expression.Parameters[i]);
}
Is this valid in c#? to add the int i into the Console.WriteLine(..) method.
I have also tried:
Console.WriteLine("Expression param[{" + i+1 +"}]: {"+ i +"}", expression.Parameters[i]);

It work's
Console.WriteLine("Expression param[{0}]: {1}", i+1, expression.Parameters[i]);
But if you are using C# 6, string interpolation is better:
Console.WriteLine($"Expression param[{i+1}]: {expression.Parameters[i]}");
String interpolation lets you more easily format strings. String.Format and its cousins are very versatile, but their use is somewhat clunky and error prone. Particularly unfortunate is the use of numbered placeholders like {0} in the format string, which must line up with separately supplied arguments.
ref: https://blogs.msdn.microsoft.com/csharpfaq/2014/11/20/new-features-in-c-6/

Use it in this way:
Console.WriteLine("Expression param[{0}]: {1}", i, expression.Parameters[i]);
{0} refers to the first argument after the format (i).
{1} refers to the second argument after the format (expression.Parameters[i]).
And so on.

TextBox string with variables

I need to be able to make a textbox string take from variables and put the variables into a string.
textBoxEmailTemplateUT.Text = "The documents are as follows: FileNameHere ► {TotalPanelCountUtah} ► {TotalkWUtah} ► ElectricalUsageOffsetHere InformationAboutTheSystemHere If There are any questions or concerns, please reply to this email. Regards YourNameHere");
This is suppose to be a type of email template after someone does a few calculations in the other code (not displayed). The {TotalPanelCountUtah} and {TotalkWUtah} are both double variables that I need to be put into the string in the textbox. Some assistance would be appreciated.

Using C# 6, you can use string interpolation, as such:
string txt = $"The values are: {value1}, {value2}";
The syntax requires that you put a $ before the string and enclose any variables within braces ({}).
Without string interpolation (pre-c#6), you can use:
string txt = String.Format("The values are: {0}, {1}", value1, value2);
This snippet of code does the same as the first one. String.Format will format a string, replacing each of the values between braces with its corresponding argument.
If:
double value1 = 0.5;
double value2 = 1.5;
then, for both of the snippets (interpolation and string.Format):
txt == "The values are: 0.5, 1.5"`

as Clay mentioned in comment use string.Format
textBoxEmailTemplateUT.Text = string.Format("The documents are as follows: FileNameHere ► {0} ► {1} ► ElectricalUsageOffsetHere InformationAboutTheSystemHere If There are any questions or concerns, please reply to this email. Regards YourNameHere",TotalPanelCountUtah, TotalkWUtah);
where TotalPanelCountUtah TotalPanelCountUtah are double type

string.Format is generally preferred, but I included a second approach using string.replace below. It is probably also a good idea to round the values before displaying them.
double val1 = 1.2345;
double val2 = 8.7654;
// recommended
textBox1.Text = string.Format(
"Value one is {0:0.00}, and value two is {1:0.00}.",
val1, val2);
// also an option
textBox1.Text = "Value one is val1, and value two is val2."
.Replace("val1", Math.Round(val1, 2).ToString())
.Replace("val2", Math.Round(val2, 2).ToString());

String.Format certain argument

I would assume this is a common question but I couldn't seem to find anything on SO or google.
Is it possible to only format a single argument. For example, format string foo = "{0} is {1} when {2}"; so that it returns read "{0} is cray when {2}"?
Intentions:
I am trying to format the string while overriding a method before it gets formatted in it's base method
Success
Got it thanks to this answer, all answers were helpful :).
This unit test worked:
string foo = String.Format("{0} is {1} when {2}", "{0}", "cray", "{2}");
Assert.AreEqual("{0} is cray when {2}", foo);
string bar = string.Format(foo, "this", null, "it works");
Assert.AreEqual("this is cray when it works", bar);

Taking your question at face value, I suppose you could do the following:
string foo = String.Format("{0} is {1} when {2}", "{0}", "cray", "{2}");
That is, simply replace each unevaluated format item with itself.

No, this is not possible. String.Format is going to try and replace every bracketed placeholder. If you do not supply the correct number of arguments, an exception will be raised.
I'm not sure why you are trying to do this, but if you want the output to look like that, you'll have to escape the brackets:
var foo = String.Format("{{0}} is {0} when {{1}}", "cray");
// foo is "{0} is cray when {1}"
Perhaps if you tell us what exactly you're trying to do, we'll be able to better help you.

Auto-Generate place holder format string for String.Format()

Is there a way to tell the String.Format() function (without writing my own function) how many placeholders there are dynamically? It we be great to say 15, and know I'd have {0}-{14} generated for me. I'm generate text files and I often have more than 25 columns. It would greatly help.
OK,
I will rephrase my question. I wanted to know if it is at all possible to tell the String.Format function at execution time how many place-holders I want in my format string without typing them all out by hand.
I'm guessing by the responses so far, I will just go ahead and write my own method.
Thanks!

You could use Enumerable.Range and LINQ to generate your message string.
Enumerable.Range(0, 7).Select(i => "{" + i + "}").ToArray()
generates following string:
"{0}{1}{2}{3}{4}{5}{6}"

Adding a bit to AlbertEin's response, I don't believe String.Format can do this for you out-of-the-box. You'll need to dynamically create the format string prior to using the String.Format method, as in:
var builder = new StringBuilder();
for(var i = 0; i < n; ++i)
{
builder.AppendFormat("{0}", "{" + i + "}");
}
String.Format(builder.ToString(), ...);
This isn't exactly readable, though.

Why use string.Format when there is no formatting (atleast from what I can see in your question)? You could use simple concatenation using stringbuilder instead.

There is a way to do this directly:
Just create a custom IFormatProvider, then use this overload of string.Format.
For example, if you want to always have 12 decimal points, you can do:
CultureInfo culture = Thread.CurrentThread.CurrentCulture.Clone(); // Copy your current culture
NumberFormatInfo nfi = culture.NumberFormat;
nfi.NumberDecimalDigits = 12; // Set to 12 decimal points
string newResult = string.Format(culture, "{0}", myDouble); // Will put it in with 12 decimal points

String has how many parameters

Before using String.Format to format a string in C#, I would like to know how many parameters does that string accept?
For eg. if the string was "{0} is not the same as {1}", I would like to know that this string accepts two parameters
For eg. if the string was "{0} is not the same as {1} and {2}", the string accepts 3 parameters
How can I find this efficiently?

String.Format receives a string argument with format value, and an params object[] array, which can deal with an arbitrary large value items.
For every object value, it's .ToString() method will be called to resolve that format pattern
EDIT: Seems I misread your question. If you want to know how many arguments are required to your format, you can discover that by using a regular expression:
string pattern = "{0} {1:00} {{2}}, Failure: {0}{{{1}}}, Failure: {0} ({0})";
int count = Regex.Matches(Regex.Replace(pattern,
#"(\{{2}|\}{2})", ""), // removes escaped curly brackets
#"\{\d+(?:\:?[^}]*)\}").Count; // returns 6
As Benjamin noted in comments, maybe you do need to know number of different references. If you don't using Linq, here you go:
int count = Regex.Matches(Regex.Replace(pattern,
#"(\{{2}|\}{2})", ""), // removes escaped curly brackets
#"\{(\d+)(?:\:?[^}]*)\}").OfType<Match>()
.SelectMany(match => match.Groups.OfType<Group>().Skip(1))
.Select(index => Int32.Parse(index.Value))
.Max() + 1; // returns 2
This also address #280Z28 last problem spotted.
Edit by 280Z28: This will not validate the input, but for any valid input will give the correct answer:
int count2 =
Regex.Matches(
pattern.Replace("{{", string.Empty),
#"\{(\d+)")
.OfType<Match>()
.Select(match => int.Parse(match.Groups[1].Value))
.Union(Enumerable.Repeat(-1, 1))
.Max() + 1;

You'll have to parse through the string and find the highest integer value between the {}'s...then add one.
...or count the number of sets of {}'s.
Either way, it's ugly. I'd be interested to know why you need to be able to figure out this number programatically.
EDIT
As 280Z28 mentioned, you'll have to account for the various idiosyncrasies of what can be included between the {}'s (multiple {}'s, formatting strings, etc.).

I rely on ReSharper to analyze that for me, and it is a pity that Visual Studio does not ship with such a neat feature.