How to pass dynamically-generated search criteria to LINQ

I have some trouble with LINQ. In my program I generate a SQL search query like

select * from emp "where empId=1 and empname='abc'"

(where the quoted text is generated in my code). I can pass the generated "where empId..." string text to the SQL query.

I'd like to do the same thing in LINQ - I want to pass this string as the search criteria i.e. something like

var employee=from a in Employee.AsEnumerable()
             "where empId=1 and empname='abc'"
          开发者_开发问答    select a;

Is this possible? Thanks in advance.

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You can take the base query (in your case Employee.AsEnumerable()) and use the logic you use to generate the string to compose a new query. For example:

if(/*your logic for generating the string "where empId=1" here*/)
{
    query = query.Where(a.empId == 1);
}

if(/*your logic for generating the string "empname='abc'" here*/)
{
    query = query.Where(a.empname == "abc");
}

The resulting query object will have all the operators composed. However as others have said this is not trivial in the general case. It is not trivial with SQL strings either. If all you need to generate are several filters it will work but if you need complex expressions it will be a problem.

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101 LINQ Samples

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It's pretty hard, unless you intend to employ:

• Dynamic code compilation, or
• You are willing to create a (very complicated) parser to analyze the query and call the respective linq extension methods

I personally have no experience in the latter. As for the former, it is a bit tricky and can go nastily wrong if you don't do proper caching and security checks. Executable code injection is very dangerous.

I think you had better use different methods to filter content using methods like Where() if the number of queries can be predetermined or return to SQL if not. Usually you don't need to do this unless the query is manually entered by the user.