相信EF大家都不陌生了，因为数据库表跟程序实体是一一对应的原因，我们能够通过lambda这种函数式的编程方式进行操作数据库，感觉非常清晰明了。与我们直接写SQL相比，lambda是强类型，拥有更好的扩展性，伸缩性，而且编程更加的方便，快捷。。下面我们就基于Expression和lambda来与大家构建一个属于自己的ORM框架。
思路的话很简单，就是将lambda转换成我们对应的数据库所需的查询条件，然后执行查询，再将结果以反射的方式封装成List<T>返回出去。
Expression
大家使用EF的时候多多少少会留意到有Expression这个东西。特别是查询时会看到要你传入Expression<Func<T,bool>>这样类型的参数，它又和Func<T,bool>有什么比同呢？
Expression<Func<T,bool>>是表达式树，我们可以通过它来分析我们的委托中的函数。当调用Compile方法后就会变成委托，才能执行。
Func<T,bool>只是一个普通的委托。
例如我们现在有个实体类Staff
public class Staff
{
public string Name { get; set; }
public int Age { get; set; }
public string Code { get; set; }
public DateTime? Birthday { get; set; }
public bool Deletion { get; set; }
}
我们还有一个这样的方法
class Program
{
static void Main(string[] args)
{
FindAs<Staff>(x => x.Code == "张三" && x.Name.Contains("张"));
}
public static List<T> FindAs<T>(Expression<Func<T, bool>> func)
{
//将func转换成对应数据库的查询条件，然后执行查询
return null;//将结果返回
}
}
我们希望通过 FindAs<Staff>(x => x.Age <50 && x.Name.Contains("张")); 就能查询出Staff表中Age<50并且Name包含有“张”字的人的信息。而生成的sql语句应该是select * from staff where Age<50 and Name like ‘%张%‘。现在我们就来分析下这个func
从上面的图我们可以看到当前的Expression是一个lambda表达式，我们点开它的body看看。
我们可以看到body里分为左边和右边，还有NodeType。和我们的lambda对比下看看‘x => x.Age <50 && x.Name.Contains("张")‘是不是找到点灵感了？我们再继续把左边和右边拆开看看。
可以看到我们需要的信息都有了，看来转换成SQL已经不是什么难事了，动手开搞了。
class Program
{
static void Main(string[] args)
{
FindAs<Staff>(x => x.Code == "张三" && x.Name.Contains("张"));
FindAs<Staff>(x => x.Age <= 12 && x.Name.Contains("张"));
Console.ReadKey();
}
public static List<T> FindAs<T>(Expression<Func<T, bool>> func)
{
BinaryExpression Binary = func.Body as BinaryExpression;
string left = ResovleFunc(Binary.Left);
string right = ResovleLinqToObject(Binary.Right);
string oper = GetOperator(Binary.NodeType);
string sql = string.Format("select * from {0} where {1}", typeof(T).Name, left + oper + right);
Console.WriteLine(sql);
return null;//将结果返回
}
//解析一般的条件，例如x=>x.name==xxxx x.age==xxx
public static string ResovleFunc(Expression express)
{
var inner = express as BinaryExpression;
string Name = (inner.Left as MemberExpression).Member.Name;
object Value = (inner.Right as ConstantExpression).Value;
var Operator = GetOperator(inner.NodeType);
string Result = string.Format("({0} {1} ‘{2}‘)", Name, Operator, Value);
return Result;
}
//解析linq to object这类扩展方法
public static string ResovleLinqToObject(Expression expression)
{
var MethodCall = expression as MethodCallExpression;
var MethodName = MethodCall.Method.Name;
if (MethodName == "Contains")
{
object Temp_Vale = (MethodCall.Arguments[0] as ConstantExpression).Value;
string Value = string.Format("%{0}%", Temp_Vale);
string Name = (MethodCall.Object as MemberExpression).Member.Name;
string Result = string.Format("{0} like ‘{1}‘", Name, Value);
return Result;
}
return null;
}
public static 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));
}
}
}
已经初步的达到了我们的目的了，但是我们的查询条件不可能固定是2个，有可能是N个，这时左边和右边又要继续再分下去，直到无法再分(想到递归了吧？)。而且我们还需要将查询条件参数化。而且我们的条件删除时也会用到。所以我们应该把它独立出来。传入一个lambda，生成sql where部分的语句，生成sqlparameter[]。这才是关键。。于是我们来构建一个解析Expresstion的类。。下面我就直接给出我自己写的实现代码了。。
/// <summary>
/// 解析lamdba
/// </summary>
public class ResolveExpress
{
public Dictionary<string, object> Argument;
public string SqlWhere;
public SqlParameter[] Paras;
/// <summary>
/// 解析lamdba，生成Sql查询条件
/// </summary>
/// <param name="expression"></param>
/// <returns></returns>
public void ResolveExpression(Expression expression)
{
this.Argument = new Dictionary<string, object>();
this.SqlWhere = Resolve(expression);
this.Paras = Argument.Select(x => new SqlParameter(x.Key, x.Value)).ToArray();
}
private string Resolve(Expression expression)
{
if (expression is LambdaExpression)
{
LambdaExpression lambda = expression as LambdaExpression;
expression = lambda.Body;
return Resolve(expression);
}
if (expression is BinaryExpression)
{
BinaryExpression binary = expression as BinaryExpression;
if (binary.Left is MemberExpression && binary.Right is ConstantExpression)//解析x=>x.Name=="123" x.Age==123这类
return ResolveFunc(binary.Left, binary.Right, binary.NodeType);
if (binary.Left is MethodCallExpression && binary.Right is ConstantExpression)//解析x=>x.Name.Contains("xxx")==false这类的
{
object value = (binary.Right as ConstantExpression).Value;
return ResolveLinqToObject(binary.Left, value, binary.NodeType);
}
if (binary.Left is MemberExpression && binary.Right is MemberExpression)//解析x=>x.Date==DateTime.Now这种
{
LambdaExpression lambda = Expression.Lambda(binary.Right);
Delegate fn = lambda.Compile();
ConstantExpression value = Expression.Constant(fn.DynamicInvoke(null), binary.Right.Type);
return ResolveFunc(binary.Left, value, binary.NodeType);
}
}
if (expression is UnaryExpression)
{
UnaryExpression unary = expression as UnaryExpression;
if (unary.Operand is MethodCallExpression)//解析!x=>x.Name.Contains("xxx")或!array.Contains(x.Name)这类
return ResolveLinqToObject(unary.Operand, false);
if (unary.Operand is MemberExpression && unary.NodeType == ExpressionType.Not)//解析x=>x.isDeletion这样的
{
ConstantExpression constant = Expression.Constant(false);
return ResolveFunc(unary.Operand, constant, ExpressionType.Equal);
}
expression = unary.Operand;
}
if (expression is MethodCallExpression)//x=>x.Name.Contains("xxx")或array.Contains(x.Name)这类
{
MethodCallExpression methodcall = expression as MethodCallExpression;
return ResolveLinqToObject(methodcall, true);
}
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, Expression right, ExpressionType expressiontype)
{
var Name = (left as MemberExpression).Member.Name;
var Value = (right as ConstantExpression).Value;
var Operator = GetOperator(expressiontype);
string CompName = SetArgument(Name, Value.ToString());
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))
{
int code = Guid.NewGuid().GetHashCode();
if (code < 0)
code *= -1;
temp = name + code;
}
Argument[temp] = value;
return temp;
}
private string In(MethodCallExpression expression, object isTrue)
{
var Argument1 = (expression.Arguments[0] as MemberExpression).Expression as ConstantExpression;
var Argument2 = expression.Arguments[1] as MemberExpression;
var Field_Array = Argument1.Value.GetType().GetFields().First();
object[] Array = Field_Array.GetValue(Argument1.Value) 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)
{
object Temp_Vale = (expression.Arguments[0] as ConstantExpression).Value;
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;
}
}
static void Main(string[] args)
{
string[] Names = { "Andy", "Amy", "Mike" };
Expression<Func<Staff, bool>> func = x => (!Names.Contains(x.Name) && (x.Name == "A" || x.Name.Count() > 5));
ResolveExpress resolve = new ResolveExpress();
resolve.ResolveExpression(func);
Console.WriteLine(resolve.SqlWhere);
foreach (var item in resolve.Paras)
{
Console.WriteLine(item.ParameterName + ":" + item.Value);
}
Console.ReadKey();
}
结果：
这里有几个重要的东西要给大家讲下
string[] Names={"Andy","Amy","Mike"};
1.）x => Names.Contains(x.Name);
2.）x => Names.Contains(x.Name)==false;
3.）x => !Names.Contains(x.Name);
这3种在Expression中的表现都不一样
1的话会看成是一个静态方法（MethodCallExpression）
2的话会看成是一个2元运算（BinaryExpression）
3的话会看成是一个1元运算（UnaryExpression）
所以我们都要支持，处理都有所不同。
还有
x=>x.Birthday<DateTime.Now;
string name="123";
x=>x.Name==name;
和
x=>x.Name=="123"
的处理也不一样。大家可以在例子中细细的看看。
这样的构造使得我们切换数据库变得非常简单。因为我们程序中的查询都是基于lambda。换了数据库只要添加一个对应的lamdba转数据库查询条件的实现就可以了。写得够多了。至于数据层怎么封装，到了这一步它已经变得没什么难度了。希望大家能从文章中有所启发和帮助
时间： 2024-08-01 08:28:35