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Is there a way to Ignore escape characters but still use string interpolation? C#

So, the string I need to implement is this, I am using .Net 4.5.2 in c# in visual studio 2019, I want the espected output to be exactly as below albeit with FIRSTNAME being replaced by a variable.
beneficiaryFirstName: \\\"FIRSTNAME\\\"
This is being used with a lot of similarly structured strings to join them together to form a large graphQL query. The problem I have is that VStudio keeps throwing up errors.
Edit : I would like to make clear, I need the \'s in the string result, and I need FIRSTNAME to be treated as a variable.
I have attempted to use this.
$#"beneficiaryFirstName: \\\"{{FIRSTNAME}}\\\""
But get told that it there's unexpected characters "" and "".
What is the best way around this?

You just need the right escaping
var FIRSTNAME = "Bob";
var pad = #"\\\";
var test1 = $"beneficiaryFirstName: \\\\\\\"{FIRSTNAME}\\\\\\\"";
var test2 = #$"beneficiaryFirstName: \\\""{FIRSTNAME}\\\""";
var test3 = $"beneficiaryFirstName: {pad}\"{FIRSTNAME}{pad}\"";
Console.WriteLine(test1);
Console.WriteLine(test2);
Console.WriteLine(test3);
Output
beneficiaryFirstName: \\\"Bob\\\"
beneficiaryFirstName: \\\"Bob\\\"
beneficiaryFirstName: \\\"Bob\\\"
Disclaimer, I am not sure if the quotes are correct in your example, they seem like they are in weird places, though that could be just how it is

you can do like this.
string FIRSTNAME = "YourName";
string temp = $"beneficiaryFirstName: {FIRSTNAME}";
Console.WriteLine(temp);
It will print
beneficiaryFirstName: YourName
You can try this string temp = $"beneficiaryFirstName: \\\\{FIRSTNAME}\\\\"; if you want outout
beneficiaryFirstName: \\YourName\\
As mentioned by the OP, you can use this to get the required output.
string temp = #$"beneficiaryFirstName: \\\{FIRSTNAME}\\\";

How can I make a string out of a complex URL address

I've been trying to make this URL a workable string in C#, but unfortunately using extra "" or "#" is not cutting it. Even breaking it into smaller strings is proving difficult. I want to be able to convert the entire address into a single string.
this is the full address:
<https://my.address.com/BOE/OpenDocument/opendoc/openDocument.jsp?iDocID=ATTPCi6c.mZInSt5o3t_Xr8&sIDType=CUID&&sInstance=Last&lsMZV_MAT="+URLEncode(""+[Material].[Material - Key])+"&lsIZV_MAT=>
I've also tried this:
string url = #"https://my.address.com/BOE/OpenDocument/opendoc/openDocument.jsp?iDocID=ATTPCi6c.mZInSt5o3t_Xr8&sIDType=CUID&&sInstance=Last&lsMZV_MAT=";
string url2 = #"+ URLEncode("" +[Material].[Material - Key]) + """"";
string url3 = #"&lsIZV_MAT=";
Any help is appreciated.

The simplest solution is put additional quotes inside string literal and use string.Concat to join all of them into single URL string:
string url = #"https://my.address.com/BOE/OpenDocument/opendoc/openDocument.jsp?iDocID=ATTPCi6c.mZInSt5o3t_Xr8&sIDType=CUID&&sInstance=Last&lsMZV_MAT=";
string url2 = #"""+URLEncode(""+[Material].[Material - Key])+""";
string url3 = #"&lsIZV_MAT=";
string resultUrl = string.Concat(url, url2, url3);
NB: You can use Equals method or == operator to check if the generated string matches with desired URL string.

This may be a bit of a workaround rather than an actual solution but if you load the string from a text file and run to a breakpoint after it you should be able to find the way the characters are store or just run it from that.
You may also have the issue of some of the spaces you've added being left over which StringName.Replace could solve if that's causing issues.
I'd recommend first checking what exactly is being produced after the third statement and then let us know so we can try and see the difference between the result and original.

You are missing the triple quotes at the beginning of url2
string url = #"https://my.address.com/BOE/OpenDocument/opendoc/openDocument.jsp?iDocID=ATTPCi6c.mZInSt5o3t_Xr8&sIDType=CUID&&sInstance=Last&lsMZV_MAT=";
string url2 = #"""+URLEncode(""+[Material].[Material - Key])+""";
string url3 = #"&lsIZV_MAT=";

I just made two updates
t&lsMZV_MAT=" to t&lsMZV_MAT="" AND
[Material - Key])+" to [Material - Key])+""
string s = #"<https://my.address.com/BOE/OpenDocument/opendoc/openDocument.jsp?iDocID=ATTPCi6c.mZInSt5o3t_Xr8&sIDType=CUID&&sInstance=Last&lsMZV_MAT=""+ URLEncode([Material].[Material - Key])+""&lsIZV_MAT=>";
Console.Write(s);
Console.ReadKey();

Variable decimal formating in string interpolation

I have looked around for this, but I'm not sure it's possible with string interpolation (I'm using VS2015).
string sequenceNumber = $"{fieldValuePrefix.ToUpper()}{separator}{similarPrefixes + 1:D4}";
Is there any way to make D4 a variable ? Some say yes, some no. Apparently, VS2015 C#6.0 is able to do it.
This works, it will return a string like WMT-0021, depending on fieldValuePrefix (WMT), separator (-) and the value of similarPrefixes (20). But I'd like the "D4" part to be a method argument instead of hardcoded in there.
Any ideas ?

You can, but you have to use explicit ToString call like this:
string format = "D4";
string sequenceNumber =
$"{fieldValuePrefix.ToUpper()}{separator}{(similarPrefixes + 1).ToString(format)}";

How to place double quotes inside the double quotes of string.IndexOf() function?

I have a String I want to get the index of the "id:" i.e the id along with the double quotes.
How I am supposed to do so inside C# string.IndexOf function?

This will get the index of the string you want:
var idx = input.IndexOf("\"id:\"");
if you wanted to pull it out you'd do something like this maybe:
var idx = input.IndexOf("\"id:\"");
var val = input.Substring(idx, len);
where len is either a statically known length or also calculated by another IndexOf statement.
Honestly, this could also be done with a Regex, and if an example were available a Regex may be the right approach because you're presumably trying to get the actual value here and it's presumably JSON you're reading.

" is an escape sequence
If you want to use a double quotation mark in your string, you should use \" instead.
For example;
int index = yourstring.IndexOf("\"id:\"");
Remember, String.IndexOf method gets zero-based index of the first occurrence of the your string.

This is a simple approach: If you know double quote is before the Id then take index of id - 1?
string myString = #"String with ""id:"" in it";
var indexOfId = myString.IndexOf("id:") - 1;
Console.WriteLine(#"Index of ""id:"" is {0}", indexOfId);

Reading between the lines, if this is a JSON string, and you have .NET 4 or higher available, you can ask .NET to deserialize the string for you rather than parsing by hand: see this answer.
Alternatively you might consider Json.NET if you're working very heavily with JSON.
Otherwise, as others note, you need to escape the quotes, so for example:
text.IndexOf("\"id:\"")
text.IndexOf(#"""id:""")
or for overengineered legiblity:
string Quoted(string text)
{
return "\"" + text + "\""; // generates unnecessary garbage
}
text.IndexOf(Quoted("id:"))

Retrieve string from the middle of a string

I have a coded string that I'd like to retrieve a value from. I realize that I can do some string manipulation (IndexOf, LastIndexOf, etc.) to pull out 12_35_55_219 from the below string but I was wondering if there was a cleaner way of doing so.
"AddedProject[12_35_55_219]0"

If you can be sure of the format of the string, then several possibilities exist:
My favorite is to create a very simple tokenizer:
string[] arrParts = yourString.Split( "[]".ToCharArray() );
Since there is a regular format to the string, arrParts will have three entries, and the part you're interested in would be arrParts[1].
If the string format varies, then you will have to use other techniques.

So in summary, if you have a pattern that you can apply to your string, the easiest is to use regular expressions, as per Guffa example.
On the other hand you have different tokens all the time to define the start and end of your string, then you should use the IndexOf and LastIndexOf combination and pass the tokens as a parameter, making the example from Fredrik a bit more generic:
string GetMiddleString(string input, string firsttoken, string lasttoken)
{
int pos1 = input.IndexOf(firsttoken) + 1;
int pos2 = input.IndexOf(lasttoken);
string result = input.Substring(pos1 , pos2 - pos1);
return result
}
And this is assuming that your tokens only happens one time in the string.

That depends on how much the string can vary. You can for example use a regular expression:
string input = "AddedProject[12_35_55_219]0";
string part = Regex.Match(input, #"\[[\d_]+\]").Captures[0].Value;

There are two methods which you may find useful, there is IndexOf and LastIndexOf with the square brackets as your parameters. With a little bit of research, you should be able to pull out the project number.

Here is a improvement from Wagner Silveira's GetMiddleString
string GetMiddleString(string input, string firsttoken, string lasttoken)
{
int pos1 = input.ToLower().IndexOf(firsttoken.ToLower()) + firsttoken.Length;
int pos2 = input.ToLower().IndexOf(lasttoken.ToLower());
return input.Substring(pos1 , pos2 - pos1);
}
And here how you use it
string data = "AddedProject[12_35_55_219]0";
string[] split = data.Split("[]".ToCharArray());
rtbHelp.Text += GetMiddleString(data, split[0], split[2]).Trim("[]".ToCharArray());//print it to my C# winForm RichTextBox Help