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Is it possible to get array length inline

I have to access Dictionary<TKey, TValue> value by key (that I get from array I'm creating inline) like so:
var someString = "1.2.3";
someDictionary[someString.Split('.').ToArray()[ /---> self.Length <---/ - 1 ]];
Question: Is it possible to get array Length inline without creating new variable and assigning array to it?

You cannot do this. You need to store intermediate value in a variable if you want to access it twice.
I see no sense in trying to do this without additional variable - at least, your approach is absolutely unreadable.
However, as I understand, by [self.Length - 1] you want to access the last value in this array.
If yes, then you can just use LINQ .Last:
var someString = "1.2.3";
someDictionary[someString.Split('.').Last()]; // someDictionary["3"]

Passing by reference to n-th element in C#

In C, if we have an array, we can pass it by reference to a function. We can also use simple addition of (n-1) to pass the reference starting from n-th element of the array like this:
char *strArr[5];
char *str1 = "I want that!\n";
char *str2 = "I want this!\n";
char *str3 = "I want those!\n";
char *str4 = "I want these!\n";
char *str5 = "I want them!\n";
strArr[0] = str1;
strArr[1] = str2;
strArr[2] = str3;
strArr[3] = str4;
strArr[4] = str5;
printPartially(strArr + 1, 4); //we can pass like this to start printing from 2nd element
....
void printPartially(char** strArrPart, char size){
int i;
for (i = 0; i < size; ++i)
printf(strArrPart[i]);
}
Resulting in these:
I want this!
I want those!
I want these!
I want them!
Process returned 0 (0x0) execution time : 0.006 s
Press any key to continue.
In C#, we can also pass reference to an object by ref (or, out). The object includes array, which is the whole array (or at least, this is how I suppose it works). But how are we to pass by reference to the n-th element of the array such that internal to the function, there is only string[] whose elements are one less than the original string[] without the need to create new array?
Must we use unsafe? I am looking for a solution (if possible) without unsafe
Edit:
I understand that we could pass Array in C# without ref keyword. Perhaps my question sounds quite misleading by mentioning ref when we talk about Array. The point why I put ref there, I should rather put it this way: is the ref keyword can be used, say, to pass the reference to n-th element of the array as much as C does other than passing reference to any object (without mentioning the n-th element or something alike)? My apology for any misunderstanding occurs by my question's phrasing.

The "safe" approach would be to pass an ArraySegment struct instead.
You can of course pass a pointer to a character using unsafe c#, but then you need to worry about buffer overruns.
Incidentally, an Array in C# is (usually) allocated on the heap, so passing it normally (without ref) doesn't mean copying the array- it's still a reference that is passed (just a new one).

Edit:
You won't be able to do it as you do in C in safe code.
A C# array (i.e. string[]) is derived from abstract type Array.
It is not only a simple memory block as it is in C.
So you can't send one of it's element's reference and start iterate from there.
But there are some solutions which will give you the same taste of course (without unsafe):
Like:
As #Chris mentioned you can use ArraySegment<T>.
As Array is also an IEnumerable<T> you can use .Skip and send the returned value. (but this will give you an IEnumerable<T> instead of an Array). But it will allow you iterate.
etc...

If the method should only read from the array, you can use linq:
string[] strings = {"str1", "str2", "str3", ...."str10"};
print(strings.Skip(1).Take(4).ToArray());

Your confusion is a very common one. The essential point is realizing that "reference types" and "passing by reference" (ref keyboard) are totally independent. In this specific case, since string[] is a reference type (as are all arrays), it means the object is not copied when you pass it around, hence you are always referring to the same object.
Modified Version of C# Code:
string[] strArr = new string[5];
strArr[0] = "I want that!\n";
strArr[1] = "I want this!\n";
strArr[2] = "I want those!\n";
strArr[3] = "I want these!\n";
strArr[4] = "I want them!\n";
printPartially(strArr.Skip(1).Take(4).ToArray());
void printPartially(string[] strArr)
{
foreach (string str in strArr)
{
Console.WriteLine(str);
}
}

Question is old, but maybe answer will be useful for someone.
As of C# 7.2 there are much more types to use in that case, ex. Span or Memory.
They allow exactly for the thing you mentioned in your question (and much more).
Here's great article about them
Currently, if you want to use them, remeber to add <LangVersion>7.2</LangVersion> in .csproj file of your project to use C# 7.2 features

How to make arrays values equal in C#?

Let's say that I have 2 string arrays with different values:
string[] sArray1 = new string[3]{"a","b","c"};
string[] sArray2 = new string[3]{"e","f","g"}
And I want to make values of sArray1 equal to values of sArray2 (I know I can write it like this) : sArray1[0] = sArray2[0]; sArray1[1]= sArray2[1]; sArray1[2]=sArray2[2];
For 3 values it's easy, but what if I had 100 values in an array? Is there any other way that I can make array values equal?
p.s. sorry for my bad English :(

Something like this (with a little error checking):
if (sArray2.Length == sArray1.Length)
{
sArray2.CopyTo(sArray1, 0);
}
Regards

I'm assuming you want to keep the reference to the original array in sArray1? Then do this:-
Array.Copy(sArray2, sArray1, sArray1.Length);

If you want them to function independently of each other than you can use .Clone() as of .NET 5.0
string[] sArray1 = (string[])sArray2.Clone();
In the above scenario if you change a value in one array it will not affect the other - this is called a "shallow copy" (AKA copy by val). If you want the values in both arrays to be tied to each other (typically not desirable) you can do a simple assignment like this:
string[] sArray1 = sArray2;
In this case if you change a value in either array the value(s) in the other array will update (AKA copy by ref).

How to assign contents of List<String> to a RichTextBox?

I've tried this:
richTextBoxResults.Text = listStrSessionIdLines.ToString();
...but get the List's ToString() representation (I guess that's what it is: "System.Collections.Generic.List`1[System.String]").
...and I've tried to try this:
listStrSessionIdLines.CopyTo(richTextBoxResults.Lines);
...but I get, "Argument Exception was unhandled. Destination array was not long enough. Check destIndex and length, and the array's lower bounds."
Does this mean I have to assign the RichTextBox a number of lines first, or...???

This works for me:
myRichTextBox.Lines = myList.ToArray();

Most classes in the BCL have a ToString() method.
When it comes to a List Of strSessionIdLines the ToString() tells you what type of object it is.
If you are casting for example an int to a string the int.ToString() will return its value, but if you do it on a array of integers int[].ToString it's ToString() method wont return eg a comma/linefeed separated string of values. As it appeared you expected.
This is why assigning the .ToArray to the .Lines property of the
RichTextBox or a loop (or aggregate) to concatenate the List Of String
into one string to suit the .Text property of the RichTextBox works.
One more tip I live by is when I need to call a method, eg String.Format I hover my mouse so that I can see what the method expects - expectsbeing the keyword. Then say the method wants a argument in the parameter thats of Type X, I declare type X and pass it in. Methods often have overloads, meaning that they can work with different parameters, so pressing up/down to scroll through them is also helpful in working out what is the most convenient in your situation. When you are passing in arguments to a method (in its parameter) type comma to refresh the tooltip indicating each arguments datatype.

Try This:
List<String> list = new List<String>();
list.Add("1");
list.Add("2");
richTextBox1.Lines = list.ToArray();

Using linq Aggregate which applies an accumulator function over a sequence.
richTextBoxResults.Text = listStrSessionIdLines.Aggregate((i, j) => i + j);

setting variable in if statement, then told it's unassigned

I want to initialize an array--I don't know how big it will be. then set it in a condition
so I've got:
string[] my_string;
if(x==2)
{
my_string=File.ReadAllLines("file.txt");
}
string new_string=my_string[1];
It's telling me I've an unassigned local variable, because it's in the condition. How do I get around this?

You need to make sure it has a value if x isn't 2.

At the moment, if x is not equal to 2, then you have no values in your array, but you're still calling that array anyway. One thing you could do is move the new_string assingment inside the if statement. Of course, this may not be the best method if you have other values of x you watch to check against. If so, a Switch..Case might be better.
string[] my_string;
//set new_string to be empty for now
string new_string = String.Empty;
if(x==2)
{
my_string=File.ReadAllLines("file.txt");
//Make sure there are at least two elements
if(my_string.Length >= 2)
//Get the second element of the array (remember, 0 is the first element)
new_string = my_string[1];
}

Why not Create a List instead and then utilize the File.ReadAllLines("file.txt")
also are you including the FilePath along with file.txt
here is a free code snippet for you to use I use this alot when I want to load a TextFile into a List as one bulk load...
List<string> lstLinesFromFile = new List<string>(File.ReadAllLines(yourFilePath+ "file.txt"));
from there you can check in the debugger or add a quickwatch to lstLinesFromFile and see all the text that was loaded. each line from there will be accessed link ordinal so use a for loop or foreach loop