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上篇文章标题乱起，被吐槽了，这次学乖了。
上篇文章中介绍了如何解析Expression生成对应的SQL语句，以及IQueryable的一些概念，以及我们所搭建的框架的思想等。但还没把它们结合并应用起来。这一篇文章将更黄更暴力，揭露IQueryable在实际使用中延迟加载的实现原理，结合上篇对Expression的解析，我们来实现一个自己的“延迟加载”
如果还不太了解如何解析Expression和IQueryable的一些基本概念，可以先看看我的上篇文章
我们先来做些基本工作，定义一个IDataBase接口，里面可以定义些查询，删除，修改，新增等方法，为了节约时间，我们就定义一个查询和删除的方法，再定义一个获取IQueryable<T>实例的方法
public interface IDataBase
{
List<T> FindAs<T>(Expression<Func<T, bool>> lambdawhere);
int Remove<T>(Expression<Func<T, bool>> lambdawhere);
IQueryable<T> Source<T>();
}
再添加一个类DBSql，实现我们上面的IDataBase接口，这个类是负责提供对sql数据库的操作
public class DBSql : IDataBase
{
public List<T> FindAs<T>(Expression<Func<T, bool>> lambdawhere)
{
throw new NotImplementedException();
}
public int Remove<T>(Expression<Func<T, bool>> lambdawhere)
{
throw new NotImplementedException();
}
public IQueryable<T> Source<T>()
{
throw new NotImplementedException();
}
}
IQueryable<T>
上篇文章中有个朋友的回复对IQueryable的解释十分到位，“IQueryable只存贮条件，不立即运行，从而可以实现延迟加载。”那它是如何存贮条件，如何延迟加载的？
这时我们为了提供 public IQueryable<T> Source<T>() 所需的对象。我们再来建一个SqlQuery类，实现IQueryable<T>。
public class SqlQuery<T> : IQueryable<T>
{
public IEnumerator<T> GetEnumerator()
{
throw new NotImplementedException();
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
throw new NotImplementedException();
}
public Type ElementType
{
get { throw new NotImplementedException(); }
}
public Expression Expression
{
get { throw new NotImplementedException(); }
}
public IQueryProvider Provider
{
get { throw new NotImplementedException(); }
}
}
看到这里大家都不陌生吧？
GetEnumerator()是IEnumerable<T>里的。有了它我们就能foreach了。有泛型和非泛型版本，所以有2个
Type提供访问当前对象的类型（反正由你定义。。。）
Expression是贮存查询条件的
IQueryProvider简单的翻译过来就是查询提供者，它是负责创建查询条件和执行查询的。我们写一个SqlProvider类来实现它
public class SqlProvider<T> : IQueryProvider
{
public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
{
throw new NotImplementedException();
}
public IQueryable CreateQuery(Expression expression)
{
throw new NotImplementedException();
}
public TResult Execute<TResult>(Expression expression)
{
throw new NotImplementedException();
}
public object Execute(Expression expression)
{
throw new NotImplementedException();
}
}
CreateQuery是创建查询条件。。
平时我们
IQueryable query=xxx源;
query=query.Where(x=>x.Name=="123");
这时Where方法里做的其实就是将前面query的Expression属性和Where里的(x=>x.Name=="123")相并，并且调用Provider属性里的CreateQuery方法。我们可以把我们的代码改成这样，来看看到底是不是这么回事。
public class DBSql : IDataBase
{
public IQueryable<T> Source<T>()
{
return new SqlQuery<T>();
}
public List<T> FindAs<T>(Expression<Func<T, bool>> lambdawhere)
{
throw new NotImplementedException();
}
public int Remove<T>(Expression<Func<T, bool>> lambdawhere)
{
throw new NotImplementedException();
}
}
public class SqlQuery<T> : IQueryable<T>
{
private Expression _expression;
private IQueryProvider _provider;
public SqlQuery()
{
_provider = new SqlProvider<T>();
_expression = Expression.Constant(this);
}
public SqlQuery(Expression expression, IQueryProvider provider)
{
_expression = expression;
_provider = provider;
}
public IEnumerator<T> GetEnumerator()
{
throw new NotImplementedException();
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
throw new NotImplementedException();
}
public Type ElementType
{
get { return typeof(SqlQuery<T>); }
}
public Expression Expression
{
get { return _expression; }
}
public IQueryProvider Provider
{
get { return _provider; }
}
}
public class SqlProvider<T> : IQueryProvider
{
public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
{
IQueryable<TElement> query = new SqlQuery<TElement>(expression, this);
return query;
}
public IQueryable CreateQuery(Expression expression)
{
throw new NotImplementedException();
}
public TResult Execute<TResult>(Expression expression)
{
throw new NotImplementedException();
}
public object Execute(Expression expression)
{
throw new NotImplementedException();
}
}
public class Staff
{
public int ID { get; set; }
public string Code { get; set; }
public string Name { get; set; }
public DateTime? Birthday { get; set; }
public bool Deletion { get; set; }
}
static void Main(string[] args)
{
IDataBase db = new DBSql();
IQueryable<Staff> query = db.Source<Staff>();
string name = "张三";
Expression express = null;
query = query.Where(x => x.Name == "赵建华");
express = query.Expression;
query = query.Where(x => x.Name == name);
express = query.Expression;
}
段点打在
public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
每次query.Where都会跑这里来。并且Expression都是前后相并的结果。
到了这一步，相信大家都明白了IQueryable只存贮条件这个概念了吧。
那延迟加载呢？什么时候加载啊！当我们foreach或者ToList/ToArray时啊。这时你想到了什么？GetEnumerator()。在调用GetEnumerator()时。我们再调用Provider里的Execute(Expression)。里面解析Expression，生成SQL语句，通过反射的方式生成实例，再一个个返回回去。完成！下面我直接给代码了。解析Expression的类我也改了，这个更黄更暴力。
public class ResolveExpression
{
public Dictionary<string, object> Argument;
public string SqlWhere;
public SqlParameter[] Paras;
private int index = 0;
/// <summary>
/// 解析lamdba，生成Sql查询条件
/// </summary>
/// <param name="expression"></param>
/// <returns></returns>
public void ResolveToSql(Expression expression)
{
this.index = 0;
this.Argument = new Dictionary<string, object>();
this.SqlWhere = Resolve(expression);
this.Paras = Argument.Select(x => new SqlParameter(x.Key, x.Value)).ToArray();
}
private object GetValue(Expression expression)
{
if (expression is ConstantExpression)
return (expression as ConstantExpression).Value;
if (expression is UnaryExpression)
{
UnaryExpression unary = expression as UnaryExpression;
LambdaExpression lambda = Expression.Lambda(unary.Operand);
Delegate fn = lambda.Compile();
return fn.DynamicInvoke(null);
}
if (expression is MemberExpression)
{
MemberExpression member = expression as MemberExpression;
string name = member.Member.Name;
var constant = member.Expression as ConstantExpression;
if (constant == null)
throw new Exception("取值时发生异常" + member);
return constant.Value.GetType().GetFields().First(x => x.Name == name).GetValue(constant.Value);
}
throw new Exception("无法获取值" + expression);
}
private string Resolve(Expression expression)
{
if (expression is LambdaExpression)
{
LambdaExpression lambda = expression as LambdaExpression;
expression = lambda.Body;
return Resolve(expression);
}
if (expression is BinaryExpression)//解析二元运算符
{
BinaryExpression binary = expression as BinaryExpression;
if (binary.Left is MemberExpression)
{
object value = GetValue(binary.Right);
return ResolveFunc(binary.Left, value, binary.NodeType);
}
if (binary.Left is MethodCallExpression && (binary.Right is UnaryExpression || binary.Right is MemberExpression))
{
object value = GetValue(binary.Right);
return ResolveLinqToObject(binary.Left, value, binary.NodeType);
}
}
if (expression is UnaryExpression)//解析一元运算符
{
UnaryExpression unary = expression as UnaryExpression;
if (unary.Operand is MethodCallExpression)
{
return ResolveLinqToObject(unary.Operand, false);
}
if (unary.Operand is MemberExpression)
{
return ResolveFunc(unary.Operand, false, ExpressionType.Equal);
}
}
if (expression is MethodCallExpression)//解析扩展方法
{
return ResolveLinqToObject(expression, true);
}
if (expression is MemberExpression)//解析属性。。如x.Deletion
{
return ResolveFunc(expression, true, ExpressionType.Equal);
}
var body = expression as BinaryExpression;
if (body == null)
throw new Exception("无法解析" + expression);
var Operator = GetOperator(body.NodeType);
var Left = Resolve(body.Left);
var Right = Resolve(body.Right);
string Result = string.Format("({0} {1} {2})", Left, Operator, Right);
return Result;
}
/// <summary>
/// 根据条件生成对应的sql查询操作符
/// </summary>
/// <param name="expressiontype"></param>
/// <returns></returns>
private string GetOperator(ExpressionType expressiontype)
{
switch (expressiontype)
{
case ExpressionType.And:
return "and";
case ExpressionType.AndAlso:
return "and";
case ExpressionType.Or:
return "or";
case ExpressionType.OrElse:
return "or";
case ExpressionType.Equal:
return "=";
case ExpressionType.NotEqual:
return "<>";
case ExpressionType.LessThan:
return "<";
case ExpressionType.LessThanOrEqual:
return "<=";
case ExpressionType.GreaterThan:
return ">";
case ExpressionType.GreaterThanOrEqual:
return ">=";
default:
throw new Exception(string.Format("不支持{0}此种运算符查找！" + expressiontype));
}
}
private string ResolveFunc(Expression left, object value, ExpressionType expressiontype)
{
string Name = (left as MemberExpression).Member.Name;
string Operator = GetOperator(expressiontype);
string Value = value.ToString();
string CompName = SetArgument(Name, Value);
string Result = string.Format("({0} {1} {2})", Name, Operator, CompName);
return Result;
}
private string ResolveLinqToObject(Expression expression, object value, ExpressionType? expressiontype = null)
{
var MethodCall = expression as MethodCallExpression;
var MethodName = MethodCall.Method.Name;
switch (MethodName)//这里其实还可以改成反射调用，不用写switch
{
case "Contains":
if (MethodCall.Object != null)
return Like(MethodCall);
return In(MethodCall, value);
case "Count":
return Len(MethodCall, value, expressiontype.Value);
case "LongCount":
return Len(MethodCall, value, expressiontype.Value);
default:
throw new Exception(string.Format("不支持{0}方法的查找！", MethodName));
}
}
private string SetArgument(string name, string value)
{
name = "@" + name;
string temp = name;
while (Argument.ContainsKey(temp))
{
temp = name + index;
index = index + 1;
}
Argument[temp] = value;
return temp;
}
private string In(MethodCallExpression expression, object isTrue)
{
var Argument1 = expression.Arguments[0];
var Argument2 = expression.Arguments[1] as MemberExpression;
var fieldValue = GetValue(Argument1);
object[] array = fieldValue as object[];
List<string> SetInPara = new List<string>();
for (int i = 0; i < array.Length; i++)
{
string Name_para = "InParameter" + i;
string Value = array[i].ToString();
string Key = SetArgument(Name_para, Value);
SetInPara.Add(Key);
}
string Name = Argument2.Member.Name;
string Operator = Convert.ToBoolean(isTrue) ? "in" : " not in";
string CompName = string.Join(",", SetInPara);
string Result = string.Format("{0} {1} ({2})", Name, Operator, CompName);
return Result;
}
private string Like(MethodCallExpression expression)
{
Expression argument = expression.Arguments[0];
object Temp_Vale = GetValue(argument);
string Value = string.Format("%{0}%", Temp_Vale);
string Name = (expression.Object as MemberExpression).Member.Name;
string CompName = SetArgument(Name, Value);
string Result = string.Format("{0} like {1}", Name, CompName);
return Result;
}
private string Len(MethodCallExpression expression, object value, ExpressionType expressiontype)
{
object Name = (expression.Arguments[0] as MemberExpression).Member.Name;
string Operator = GetOperator(expressiontype);
string CompName = SetArgument(Name.ToString(), value.ToString());
string Result = string.Format("len({0}){1}{2}", Name, Operator, CompName);
return Result;
}
}
public interface IDataBase
{
List<T> FindAs<T>(Expression<Func<T, bool>> lambdawhere);
int Remove<T>(Expression<Func<T, bool>> lambdawhere);
IQueryable<T> Source<T>();
}
namespace Data.DataBase
{
public class DBSql : IDataBase
{
private readonly static string ConnectionString = @"Data Source=.;Initial Catalog=btmmcms-Standard;Persist Security Info=True;User ID=sa;Password=sa;";
public IQueryable<T> Source<T>()
{
return new SqlQuery<T>();
}
public List<T> FindAs<T>(Expression<Func<T, bool>> lambdawhere)
{
using (SqlConnection Conn = new SqlConnection(ConnectionString))
{
using (SqlCommand Command = new SqlCommand())
{
try
{
Command.Connection = Conn;
Conn.Open();
string sql = string.Format("select * from {0}", typeof(T).Name);
if (lambdawhere != null)
{
ResolveExpression resolve = new ResolveExpression();
resolve.ResolveToSql(lambdawhere);
sql = string.Format("{0} where {1}", sql, resolve.SqlWhere);
Command.Parameters.AddRange(resolve.Paras);
}
//为了测试，就在这里打印出sql语句了
Console.WriteLine(sql);
Command.CommandText = sql;
SqlDataReader dataReader = Command.ExecuteReader();
List<T> ListEntity = new List<T>();
while (dataReader.Read())
{
var constructor = typeof(T).GetConstructor(new Type[] { });
T Entity = (T)constructor.Invoke(null);
foreach (var item in Entity.GetType().GetProperties())
{
var value = dataReader[item.Name];
if (value == null)
continue;
if (value is DBNull)
value = null;
item.SetValue(Entity, value, null);
}
ListEntity.Add(Entity);
}
if (ListEntity.Count == 0)
return null;
return ListEntity;
}
catch (Exception ex)
{
throw ex;
}
finally
{
Conn.Close();
}
}
}
}
public int Remove<T>(Expression<Func<T, bool>> lambdawhere)
{
throw new NotImplementedException();
}
}
public class SqlQuery<T> : IQueryable<T>
{
private Expression _expression;
private IQueryProvider _provider;
public SqlQuery()
{
_provider = new SqlProvider<T>();
_expression = Expression.Constant(this);
}
public SqlQuery(Expression expression, IQueryProvider provider)
{
_expression = expression;
_provider = provider;
}
public IEnumerator<T> GetEnumerator()
{
var result = _provider.Execute<List<T>>(_expression);
if (result == null)
yield break;
foreach (var item in result)
{
yield return item;
}
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
throw new NotImplementedException();
}
public Type ElementType
{
get { return typeof(SqlQuery<T>); }
}
public Expression Expression
{
get { return _expression; }
}
public IQueryProvider Provider
{
get { return _provider; }
}
}
public class SqlProvider<T> : IQueryProvider
{
public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
{
IQueryable<TElement> query = new SqlQuery<TElement>(expression, this);
return query;
}
public IQueryable CreateQuery(Expression expression)
{
throw new NotImplementedException();
}
public TResult Execute<TResult>(Expression expression)
{
MethodCallExpression methodCall = expression as MethodCallExpression;
Expression<Func<T, bool>> result = null;
while (methodCall != null)
{
Expression method = methodCall.Arguments[0];
Expression lambda = methodCall.Arguments[1];
LambdaExpression right = (lambda as UnaryExpression).Operand as LambdaExpression;
if (result == null)
{
result = Expression.Lambda<Func<T, bool>>(right.Body, right.Parameters);
}
else
{
Expression left = (result as LambdaExpression).Body;
Expression temp = Expression.And(right.Body, left);
result = Expression.Lambda<Func<T, bool>>(temp, result.Parameters);
}
methodCall = method as MethodCallExpression;
}
var source = new DBSql().FindAs<T>(result);
dynamic _temp = source;
TResult t = (TResult)_temp;
return t;
}
public object Execute(Expression expression)
{
throw new NotImplementedException();
}
}
}
搞定，这时可以改下数据库连接，连到自己的数据库，然后像下面这样，添加一个实体类（要与数据库表对应），就可以使用了
class Program
{
public class Staff
{
public int ID { get; set; }
public string Code { get; set; }
public string Name { get; set; }
public DateTime? Birthday { get; set; }
public bool Deletion { get; set; }
}
static void Main(string[] args)
{
IDataBase db = new DBSql();
IQueryable<Staff> query = db.Source<Staff>();
query = query.Where(x => x.Name == "张三");
foreach (var item in query)
{
}
}
}
是不是很简单？
虽然信息量有点大，但慢慢理清并消化，我相信会对你又很大帮助！
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原文地址：http://www.cnblogs.com/zhangxiaolei521/p/5552175.html
