Pass string value to SQL query where in ('a,'b','c') in C sharp

I have made a windows application using C sharp, where I retrieve data from SQL database table.
I'm trying to retrieve data based on a query and for the where criteria I pass the value from the form checkedbox.
So i save the value of the checkedbox selected value to string variable Temp, the value of Temp for example is  'a','b','c'
So trying to pull all inv no where is either a, b or c
I'm trying to pass this value from Temp to the SQL query in the C # code like this

sql = "select  CusNo, InvNo,amt  from TestTbl where  Invno ( in ( '"+Temp+"') )   ";

I do not get any rows returned. On the other hand If I trying giving the value directly it pulls the desired result
so when I try
sql = "select  CusNo, InvNo,amt  from TestTbl where  Invno ( in  ('a','b','c')   ";
I get the results.
I will appreciate, if somebody can tell me the correct way to pass the string variable Temp value for the OR in the where clause.
Thank you,
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Ryan ChongBusiness Systems Analyst , ex-Senior Application EngineerCommented:

sql = "select  CusNo, InvNo,amt  from TestTbl where  Invno ( in ( "+Temp+" ) )   ";

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Eric FlammOwnerCommented:
Ryan's answer has an extra set of parentheses which will cause a syntax error. Assuming your checkbox value is exactly as stated, with the single quote marks, you could use the string object's format function rather than concatenation:
sql=String.format("Select CusNo, InvNo, amt from TestTbl where InvNo in ({0})",Temp);

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THe nice thing about this approach is you can easily add more parameters (in curly brackets) to handle various cases.
PortletPaulEE Topic AdvisorCommented:
select  CusNo, InvNo, amt  from TestTbl where  Invno ( in  ('a','b','c')   )

Just a small note, but there is no need for 2 sets parentheses

this is sufficient:
         select  CusNo, InvNo, amt  from TestTbl where  Invno  in  ('a','b','c')
Ryan ChongBusiness Systems Analyst , ex-Senior Application EngineerCommented:

sql = "select  CusNo, InvNo,amt  from TestTbl where  Invno in ( "+Temp+"  )   ";
käµfm³d 👽Commented:
Do note that using string concatenation to build SQL queries leaves your code vulnerable to SQL Injection attacks.
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