it’s not C#, it’s SQL.
in SQL you can use like
clause.
ex: select * from tblcustomer where CustomerName like 'A%'
it will display customer name start with letter A
EDIT
DECLARE @CustomerName varchar(200) = NULL
SELECT TOP 100 * FROM tblCustomer
WHERE CustomerName like CASE WHEN @CustomerName IS NULL THEN '%' ELSE @CustomerName + '%' END
1
solved Filter Name with Starting Letter