概述
前面章节，总结了Lambda树的构建，那么怎么解析Lambda表达式树那？Lambda表达式是一种委托构造而成，如果能够清晰的解析Lambda表达式树，那么就能够理解Lambda表达式要传递的正式意图。解析Lambda表达式树意义很大，比如我们用的EF框架、Rafy框架，里面封装了大量的Lambda查询表达式，通过解析表达式转换成SQL语句，即可以查询数据库，将数据呈现给前台用户；
Lambda表达式树解析
下面代码块是标识Express各个节点的信息，ExpressionType.Call标识表达式中含有方法调用，常用到该类型的3个参数为:Method.DeclaringType和Argument和object。ExpressionType.Quote标识表达式中含有一元表达式，常用到的参数是Operand,里面放置了Lamble具体表达式。
public string AnalysisExpression(Expression exp)
{
string TextSql = "";
switch (exp.NodeType)
{
case ExpressionType.Call:
{
MethodCallExpression mce = exp as MethodCallExpression;
Console.WriteLine("The Method Is {0}", mce.Method.Name);
Console.WriteLine("The Method TypeOf {0}", mce.Method.DeclaringType);
if (mce.Method.DeclaringType == typeof(string))
{
break;
}
if (mce.Method.DeclaringType != typeof(Queryable)){
break;
}
for (int i = 0; i < mce.Arguments.Count; i++)
{
TextSql+=AnalysisExpression(mce.Arguments[i]);
}
}
break;
case ExpressionType.Quote:
{
UnaryExpression ue = exp as UnaryExpression;
TextSql += AnalysisExpression(ue.Operand);
}
break;
case ExpressionType.Lambda:
{
LambdaExpression le = exp as LambdaExpression;
AnalysisExpression(le.Body);
TextSql+= ExpressTreeAnalysis.GetSqlByExpression(le.Body);
}
break;
case ExpressionType.AndAlso:
case ExpressionType.OrElse:
case ExpressionType.Equal:
{
BinaryExpression be = exp as BinaryExpression;
Console.WriteLine("The Method Is {0}", exp.NodeType.ToString());
TextSql += AnalysisExpression(be.Left);
TextSql += AnalysisExpression(be.Right);
}
break;
case ExpressionType.Constant:
{
ConstantExpression ce = exp as ConstantExpression;
Console.WriteLine("The Value Type Is {0}", ce.Value.ToString());
}
break;
case ExpressionType.Parameter:
{
ParameterExpression pe = exp as ParameterExpression;
Console.WriteLine("The Parameter Is {0}", pe.Name);
}
break;
default:
{
Console.Write("UnKnow");
}
break;
}
return TextSql;
}
由于自定义集合实现IQueryable和IQueryProvider来自定义查询项
实现IQuable定义的类：
public class BlogQueryable<T> : IQueryable<T>
{
BlogQueryProvider provider;
Expression expression;
public BlogQueryable(BlogQueryProvider provider)
{
if (provider == null)
{
throw new ArgumentNullException("provider");
}
this.provider = provider;
this.expression = Expression.Constant(this);
}
public BlogQueryable(BlogQueryProvider provider, Expression expression)
{
this.provider = provider;
this.expression = expression;
}
public Type ElementType
{
get
{
return typeof(T);
}
}
public Expression Expression
{
get
{
return expression;
}
}
public IQueryProvider Provider
{
get
{
return provider;
}
}
public IEnumerator<T> GetEnumerator()
{
return ((IEnumerable<T>)this.provider.Execute(this.expression)).GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return ((IEnumerable)this.provider.Execute(this.expression)).GetEnumerator();
}
public void Where2(Expression<Func<T, bool>> func)
{
string TextSql = ExpressTreeAnalysis.GetSqlByExpression(func.Body);
}
}
实现IQueryProvider接口的类，CreateQuery解析表达式树而Execute则是执行解析后的SQL语句，查询数据，填充list集合：
public class BlogQueryProvider : IQueryProvider
{ public IQueryable CreateQuery(Expression expression) { Type elementType = expression.Type; try { return (IQueryable)Activator.CreateInstance(typeof(BlogQueryable<>).MakeGenericType(elementType), new object[] { this, expression }); } catch(TargetInvocationException tie) { throw tie.InnerException; } } public IQueryable<TElement> CreateQuery<TElement>(Expression expression) { //string TextSql= ExpressTreeAnalysis.GetSqlByExpression(((Expression)(expression as MethodCallExpression).Arguments[1]).bo); //Console.WriteLine(TextSql); AnalysisExpression(expression); return new BlogQueryable<TElement>(this, expression); } public object Execute(Expression expression) { return this.Execute2<object>(expression); } public TResult Execute<TResult>(Expression expression) { return (TResult)this.Execute2<TResult>(expression); } public TResult Execute2<TResult>(Expression expression) { return (TResult)Activator.CreateInstance(typeof(TResult)); } }
具体下面即可使用查询
public void MyTestMethod()
{
Catagory cag = new Catagory("四大名著点评");
IList<Blog> Blogs = new List<Blog>();
Blog bok = new Blog() { Title = "红楼梦", Conent = "红楼梦书籍不错",
User = new Author() { UserName = "曹雪芹", BirefInfor = "中国清代著名作家" } };
bok.Catagories.Add(cag);
bok.Catagories.Add(new Catagory("红楼梦专栏"));
Blogs.Add(bok);
Blog bok2 = new Blog() { Title = "三国演义", Conent = "三国鼎立时期的描述",
User = new Author() { UserName = "罗贯中", BirefInfor = "中国元末明初著名作家" } };
bok2.Catagories.Add(cag);
bok2.Catagories.Add(new Catagory("三国演义专栏"));
Blogs.Add(bok2);
Blog bok3 = new Blog() { Title = "西游记", Conent = "西游记讲述四个徒弟和一个师傅取经事情",
User = new Author() { UserName = "吴承恩", BirefInfor = "中国明代杰出的小说家" } };
bok2.Catagories.Add(cag);
bok2.Catagories.Add(new Catagory("西游记专栏"));
Blogs.Add(bok3);
Blog bok4 = new Blog() { Title = "水浒传", Conent = "描述梁山好汉劫富济贫的故事",
User = new Author() { UserName = "施耐庵", BirefInfor = "中国明代杰出的小说家" } };
bok2.Catagories.Add(cag);
bok2.Catagories.Add(new Catagory("水浒传专栏"));
Blogs.Add(bok4);
BlogQueryProvider Provider = new BlogQueryProvider();
BlogQueryable<Blog> Quble = new BlogQueryable<Blog>(Provider);
var t2 = Quble.Where(p => p.Title == "123" && p.Conent.Contains("水浒"));
}
后续：针对解析Lambda表达式树成SQL，本人也是刚刚入门，至于完全要解析成SQL，还需要研究......
https://www.cnblogs.com/xibei666/p/6858299.html