Vaishali Tanwar
Vaishali Tanwar
#TECH

Dynamic Expression Evaluation using LINQ and Lambda Expressions

There are times when we need to evaluate an expression at run time. We can either write a custom parser or use .NET Compilers which will compile the code at runtime. But these compilers might take a significant amount of time to compile the code. A parser can parse the dynamic expression and further generate the result of the expression.

Consider we have a Dictionary and a DataTable. I have an expression that requires a KeyPairValue to be evaluated against a DataRow object. System.Linq.Dynamic and System.Linq.Expressions are the assemblies you all need.

Code Snippet
  1. Dictionary<string, string> dictionary = new Dictionary<string, string>();
  2. dictionary.Add(“Name”, “MyName”);
  4. DataTable dataTable = new DataTable(“Employee”);
  5. dataTable.Columns.Add(“Id”, typeof(Int64));
  6. dataTable.Columns.Add(“Name”, typeof(string));
  7. dataTable.Columns.Add(“Designation”, typeof(string));
  9. DataRow dr = dataTable.NewRow();
  10. dr[“Id”] = 1111;
  11. dr[“Name”] = “MyName”;
  12. dr[“Designation”] = “Accountant”;
  13. dataTable.Rows.Add(dr);
  15. DataRow dr2 = dataTable.NewRow();
  16. dr2[“Id”] = 1112;
  17. dr2[“Name”] = “YourName”;
  18. dr2[“Designation”] = “Receptionist”;
  19. dataTable.Rows.Add(dr2);
  21. var results = (from table in dataTable.AsEnumerable()
  22.                join dt in dataTable.AsEnumerable() on table.Field<Int64>(“Id”) equals 1111
  23.                select new { table = table, dictionary = dictionary }).AsQueryable();
  25. var ListSourceObjects = results.ToList();
  27. Dictionary<string, object> symbols = new Dictionary<string, object>();
  29. symbols.Add(“employeeTable”, ListSourceObjects[0].table);
  30. symbols.Add(“employeeList”, ListSourceObjects[0].dictionary);
  32. string strExpression = @”employeeTable[“”Name””].ToString() == employeeList[“”Name””]”;
  34. var body = System.Linq.Dynamic.DynamicExpression.Parse(typeof(bool), strExpression, symbols);
  35. LambdaExpression e = Expression.Lambda(body);
  36. var result = e.Compile().DynamicInvoke();

 

You can provide any entity, DataSet, DataTable for the result set and just query over the objects. You can use this result set to fetch the values.