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This article mainly introduces "how to convert Expression trees into SQL and sentences". In daily operations, I believe that many people have doubts about how to convert Expression trees into SQL and sentences. Xiaobian consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "how to convert Expression trees into SQL and sentences". Next, please follow the editor to study!

This happens when you encounter some parameters (not specific values) when parsing with Expression.

What we want here is id=10, not id=m_id;, so how do we parse these parameters?

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First of all, the parameter is divided into 2, one is n.id, this can be said to be a formal parameter, there is no specific data is just an attribute in the generic, this should be parsed into the string id, the other is m_id, this is worth it, we want it to resolve to a specific value (10) rather than m_id. Received by the MemberExpression class in a subclass of Expression. Let's take a look at MemberExpression's information.

1 public class StudentHelper2 {3 public int id1 {get; set;} / / attribute 4 public int id2; / / Field 5 public static int id3 {get {return 10;}} / / static attribute 6 public static int id4 = 20; / / static Field 7 8}

A data class

1 public static void Main (string [] args) 2 {3 StudentHelper h = new StudentHelper (); 4 h.id1 = 1; 5 h.id2 = 2; 6 Expression la1 = n = > n.id = = h.id1; 7 Expression la2 = n = > n.id = = h.id2; 8 Expression la3 = n = > n.id = = StudentHelper.id3 9 Expression la4 = n = > n.id = = StudentHelper.id4;10 Expression la5 = n = > n.id = = n.neworiginalist11 test (la1, "attribute"); 12 test (la2, "Field"); 13 test (la3, "static property"); 14 test (la4, "static Field") 15 test (la5, "own parameters"); 16 Console.ReadLine (); 17} 18 public static void test (Expression la,string name) 19 {20 Console.WriteLine ("\ n\ n*" + name+ "*") 21 BinaryExpression B_exp = la.Body as BinaryExpression;22 Console.WriteLine ("Expression class name:" + B_exp.Right.GetType () .Name); 23 MemberExpression m_exp = B_exp.Right as MemberExpression;24 string valueClassName = string.Empty;25 if (m_exp.Expression = = null) 26 {27 Console.WriteLine ("data is empty") 28} 29 else30 {31 valueClassName = m_exp.Expression.GetType (). Name;32 Console.WriteLine ("data Expression class name:" > debugging result: through the debugging result, you can see that the data object of its own parameter is type TypedParameterExpression, not the fields and attributes that may be static in 2, whose data object is empty, or the FieldExpression type. After judgment, the expression can be compiled as a delegate. The specific code is as follows: 1 public static object Eval (MemberExpression member) 2 {3 UnaryExpression cast = Expression.Convert (member, typeof (object)); 4 object obj = Expression.Lambda (cast). Compile (). Invoke (); 5 return obj 6} finally embed the code into: the debugging result is this, the study on "how to convert Expression trees into SQL and statements" is over, hoping to solve everyone's doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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