You can get the accompanying source from the presentation here (includes solution and project, just browse upward in the directory).
In the 10 examples you learn how to
1. Create a query
2. Create a query and have it produce a result
3. Treat an expression predicate as a value
4. Compile an expression tree to IL
5. Use LINQ to objects
6. Build a dynamic expression tree
7. Compose Queries
Possible future additions : Discuss yield keyword, discuss LINQKit for composability
Code for main program embedded below as well
1: using System;
2: using System.Collections.Generic;
3: using System.Linq;
4: using System.Text;
5: using System.Threading.Tasks;
6: using System.Linq.Expressions;
7:
8: namespace PresentationV12
9: {
10: class Program
11: {
12: static void Main(string[] args)
13: {
14: #region Example 1 LINQ Query
15: using (var repository = new Database1Entities())
16: {
17: var query = repository.Users.Where(user => user.UserId == 1);
18: }
19: #endregion
20:
21: #region Example 2 LINQ With Result
22:
23: using (var repository = new Database1Entities())
24: {
25: var usr = repository.Users.Where(user => user.Username.EndsWith("Brian")).FirstOrDefault();
26: }
27: #endregion
28:
29: #region Example 3 Expression
30:
31: using (var repository = new Database1Entities())
32: {
33: Expression<Func<User, bool>> expression = user => user.UserId != 5;
34:
35: //Entity framework does similar parsing to the below and translates the expression to SQL
36: ParameterExpression param = (ParameterExpression)expression.Parameters[0];
37: BinaryExpression operation = (BinaryExpression)expression.Body;
38: var left = operation.Left;
39: var right = operation.Right;
40:
41: var usr = repository.Users.Where(expression).FirstOrDefault();
42: }
43: #endregion
44:
45: #region Example 4 Not using Expressions - LINQ To objects
46: using (var repository = new Database1Entities())
47: {
48: //Extension for IQueryable
49: var usrs = repository.Users.Where(user => user.UserId != 5).ToList();
50:
51: //Different extension method, IEnumerable.
52: var user2ndQuery = usrs.Where(user => user.UserId != 6);
53: }
54: #endregion
55:
56: #region Example 5 Compiling Expression Tree
57:
58: using (var repository = new Database1Entities())
59: {
60: Expression<Func<User, bool>> expression = user => user.UserId != 5;
61:
62: var usrs = repository.Users.ToList();
63:
64: //Can't query objects with Expression<Func<User, bool>> expression
65: //Error - var user2ndQuery = usrs.Where(expression);
66:
67: //But you can reuse that expression like this to do that v
68: var compiledExpression = expression.Compile();
69: var user2ndQuery = usrs.Where(compiledExpression);
70: }
71: #endregion
72:
73: #region Example 6 Building a dynamic Expression tree
74:
75: var param1 = Expression.Parameter(typeof(User), "user");
76: var prop = Expression.Property(param1, "UserId");
77: var Right = Expression.Constant(5);
78: BinaryExpression expr = Expression.MakeBinary(ExpressionType.NotEqual, prop, Right);
79:
80: var userExpression = Expression.Lambda<Func<User, bool>>(
81: expr,
82: param1
83: ).Compile();
84:
85: using (var repository = new Database1Entities())
86: {
87: var usr = repository.Users.Where(userExpression).ToList();
88: }
89:
90: #endregion
91:
92: #region My Number 1 Tip for writing good LINQ
93: //Compose your queries so you don't repeat yourself
94: //Lack of easy composability in SQL is the worst crux of writing SQL
95: //If you're copying and pasting a lot, you're doing it wrong
96: using (var repository = new Database1Entities())
97: {
98: var userQuery = repository.Users
99: .Where(UserNotTwo())
100: .Where(UserNotBrian()).Where(NewMethod());
101:
102: var usrList = userQuery.ToList(); //lazy evaluation "combines" both criteria
103: }
104: #endregion
105:
106: #region SelectMany Flattening
107: //Flattens collections of collections
108: //Fairly common for a collection to contain collections with navigation properties in Entity Framework
109: //Necessary because there's no looping in a single LINQ expression--You'd have to bring the data down and loop yourself
110: //http://blog.falafel.com/Blogs/adam-anderson/2010/06/29/Flatten_Nested_Loops_with_LINQ_and_SelectMany
111:
112: using (var repository = new Database1Entities())
113: {
114: var complexList = new List<List<string>>() { new List<string> { "test", "test2" }, new List<string> { "test3", "test4" } };
115: var easyList = complexList.SelectMany(lst => lst);
116:
117: var superComplexList = new List<List<object>>() { new List<object> { "test", "test2", 25 }, new List<object> { "test3", "test4", new List<string>{ "test"} } };
118: var kindOfEasyList = superComplexList.SelectMany(lst => lst);
119: }
120: #endregion
121: }
122:
123: private static Expression<Func<User, bool>> NewMethod()
124: {
125: return tt => tt.Email != "brian.rosamilia@gmail.com";
126: }
127:
128: #region Helper Functions
129: private static Expression<Func<User, bool>> UserNotBrian()
130: {
131: return usr => usr.FirstName != "brosamilia@example.com";
132: }
133:
134: private static Expression<Func<User, bool>> UserNotTwo()
135: {
136: return user => user.UserId != 2;
137: }
138: #endregion
139: }
140: }
