正文
【手撸一个ORM】第六步、对象表达式解析和Select表达式解析
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说明
一个Orm自然不仅仅包含条件表达式，还会有如下的场景：
OrderBy(s => s.StudentName)
Select<StudentDto>(s => new StudentDto { s.Id, s.Name, SchoolName = s.School.Name})
而应用场景的不同，导致解析的方式也有所不同，在这里我们又定义了两个解析类：[ObjectMemberVisitor] 和 [SelectExpressionResolver] 。
[ ObjectMemberVisitor ] 主要用于从表达式中解析出参数的属性名，会自动忽略导航属性
[ SelectExpressionResolver ] 主要用于查询的Select方法，也是用于从表达式中解析出属性名，与ObjectMemberVisitor不同的是它不会忽略导航属性。
从下面的代码可以看出，两个类虽然功能类似，但是代码差异很大，主要是因为ObjectMemberVisitor的使用场景比较简单，只需要拿到表达式的Member(成员)就可以了，不必考虑太多。但SelectExpressionResolver不同，其解析结果需要反馈给查询工具更多信息，包括Member与Parameter的映射关系等。
对象表达式解析
using System.Collections.Generic;
using System.Linq.Expressions; namespace MyOrm.Expressions
{
public class ObjectMemberVisitor : ExpressionVisitor
{
private readonly List<string> _propertyList; public ObjectMemberVisitor()
{
_propertyList = new List<string>();
} public List<string> GetPropertyList()
{
return _propertyList;
} public void Clear()
{
_propertyList.Clear();
} protected override Expression VisitMember(MemberExpression node)
{
if (node.Expression != null && node.Expression.NodeType == ExpressionType.Parameter)
{
_propertyList.Add(node.Member.Name);
}
return node;
} protected override Expression VisitNew(NewExpression node)
{
foreach (var arg in node.Arguments)
{
if (arg.NodeType == ExpressionType.MemberAccess)
{
var member = (MemberExpression) arg;
if (member.Expression != null && member.Expression.NodeType == ExpressionType.Parameter)
{
_propertyList.Add(member.Member.Name);
}
}
}
return node;
}
}
}
Select表达式解析
using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using System.Text;
using MyOrm.Reflections; namespace MyOrm.Expressions
{
public class SelectExpressionResolver
{
private readonly List<string> _propertyList; private readonly List<SelectResolveResult> _dict; private Type _targetType; public SelectExpressionResolver()
{
_propertyList = new List<string>();
_dict = new List<SelectResolveResult>();
} public List<SelectResolveResult> GetPropertyList()
{
return _dict;
} public Type GetTargetType()
{
return _targetType;
} public void Clear()
{
_propertyList.Clear();
} public void Visit(LambdaExpression expression)
{
if (expression.Body.NodeType == ExpressionType.MemberAccess)
{
VisitMember((MemberExpression)expression.Body);
}
else if (expression.Body.NodeType == ExpressionType.MemberInit)
{
VisitMemberInit((MemberInitExpression)expression.Body);
}
else if(expression.Body.NodeType == ExpressionType.New)
{
VisitNew((NewExpression)expression.Body);
}
} protected Expression VisitMember(MemberExpression node)
{
var rootType = node.GetRootType(out var stack);
if (rootType == ExpressionType.Parameter)
{
if (stack.Count == )
{
var propertyName = stack.Pop();
var memberName = node.Member.Name; _dict.Add(new SelectResolveResult
{
PropertyName = propertyName,
MemberName = memberName,
FieldName = ""
});
}
else if (stack.Count == )
{
var propertyName = stack.Pop();
var fieldName = stack.Pop();
var memberName = node.Member.Name;
_dict.Add(new SelectResolveResult
{
MemberName = memberName,
PropertyName = propertyName,
FieldName = fieldName
});
}
}
return node;
} protected Expression VisitNew(NewExpression node)
{
_targetType = node.Type;
Console.WriteLine(_targetType);
if (node.Members != null)
{
for (var i = ; i < node.Members.Count; i++)
{
if (node.Arguments[i].NodeType == ExpressionType.MemberAccess)
{
var member = (MemberExpression) node.Arguments[i];
var rootType = member.GetRootType(out var stack);
if (rootType == ExpressionType.Parameter)
{
if (stack.Count == )
{
var propertyName = stack.Pop();
var memberName = node.Members[i].Name; _dict.Add(new SelectResolveResult
{
PropertyName = propertyName,
MemberName = memberName,
FieldName = ""
});
}
else if (stack.Count == )
{
var propertyName = stack.Pop();
var fieldName = stack.Pop();
var memberName = node.Members[i].Name;
_dict.Add(new SelectResolveResult
{
PropertyName = propertyName,
MemberName = memberName,
FieldName = fieldName
});
}
}
}
}
} return node;
} protected void VisitMemberInit(MemberInitExpression node)
{
foreach (var binding in node.Bindings)
{
var result = new SelectResolveResult { MemberName = binding.Member.Name };
if (binding.BindingType == MemberBindingType.Assignment)
{
var expression = ((MemberAssignment) binding).Expression;
if (expression.NodeType == ExpressionType.MemberAccess)
{
var member = (MemberExpression)expression;
var rootType = member.GetRootType(out var stack);
if (rootType == ExpressionType.Parameter)
{
if (stack.Count == )
{
var propertyName = stack.Pop();
var memberName = binding.Member.Name; _dict.Add(new SelectResolveResult
{
PropertyName = propertyName,
MemberName = memberName,
FieldName = ""
});
}
else if (stack.Count == )
{
var propertyName = stack.Pop();
var fieldName = stack.Pop();
var memberName = binding.Member.Name;
_dict.Add(new SelectResolveResult
{
PropertyName = propertyName,
MemberName = memberName,
FieldName = fieldName
});
}
}
}
}
}
} private string ResolveStackToField(Stack<string> parameterStack)
{
switch (parameterStack.Count)
{
case :
{
// 调用了导航属性
var propertyName = parameterStack.Pop();
var propertyFieldName = parameterStack.Pop(); return $"{propertyName}.{propertyFieldName}";
}
case :
{
var propertyName = parameterStack.Pop();
return propertyName;
}
default:
throw new ArgumentException("尚未支持大于2层属性调用。如 student.Clazz.School.Id>10，请使用类似 student.Clazz.SchoolId > 0 替代");
}
}
} public class SelectResolveResult
{
public string MemberName { get; set; } public string PropertyName { get; set; } public string FieldName { get; set; }
}
}
