如何将谓词表达式分解为查询? [英] How do I decompose a Predicate Expression into a query?
问题描述
我有以下类Person
,带有自定义的Where
方法:
I have the following class Person
, with a custom Where
method:
public class Person
{
public string Name { get; set; }
public int Age { get; set; }
public string Where(Expression<Func<Person, bool>> predicate)
{
return String.Empty;
}
}
如何以可枚举的方式检索Expression
参数名称和值?
How can I retrieve the Expression
parameter names and values in an enumerable manner?
Person p = new Person();
p.Where(i => i.Name == "Shlomi" && i.Age == 26);
为了构建一个字符串查询,并根据表达式的名称和值附加参数.
For the purpose of building a string query with parameters attached according to the name and value of the expression.
// Eventually I will convert the above into the following:
string Query = "select * from person where name = @Name AND Age = @Age";
SqlParameter[] param = new SqlParameter[] {
new SqlParameter("@Name","Shlomi"),
new SqlParameter("@Age","26")
};
推荐答案
我当然认为我的先前答案.
I certainly think you should follow StriplingWarrior's advice and use LINQ to Entities or LINQ to SQL, but for the sake of reinventing the wheel (poorly) I'll build off of a prior answer of mine.
// Start with a method that takes a predicate and retrieves the property names
static IEnumerable<string> GetColumnNames<T>(Expression<Func<T,bool>> predicate)
{
// Use Expression.Body to gather the necessary details
var members = GetMemberExpressions(predicate.Body);
if (!members.Any())
{
throw new ArgumentException(
"Not reducible to a Member Access",
"predicate");
}
return members.Select(m => m.Member.Name);
}
现在,您需要遍历表达式树,访问每个候选表达式,并确定其中是否包含MemberExpression
.下面的GetMemberExpressions
方法将遍历表达式树并检索在以下位置找到的每个MemberExpression
:
Now you need to walk the expression tree, visiting each candidate expression, and determine if it includes a MemberExpression
. The GetMemberExpressions
method below will walk an expression tree and retrieve each of the MemberExpression
s found within:
static IEnumerable<MemberExpression> GetMemberExpressions(Expression body)
{
// A Queue preserves left to right reading order of expressions in the tree
var candidates = new Queue<Expression>(new[] { body });
while (candidates.Count > 0)
{
var expr = candidates.Dequeue();
if (expr is MemberExpression)
{
yield return ((MemberExpression)expr);
}
else if (expr is UnaryExpression)
{
candidates.Enqueue(((UnaryExpression)expr).Operand);
}
else if (expr is BinaryExpression)
{
var binary = expr as BinaryExpression;
candidates.Enqueue(binary.Left);
candidates.Enqueue(binary.Right);
}
else if (expr is MethodCallExpression)
{
var method = expr as MethodCallExpression;
foreach (var argument in method.Arguments)
{
candidates.Enqueue(argument);
}
}
else if (expr is LambdaExpression)
{
candidates.Enqueue(((LambdaExpression)expr).Body);
}
}
}
这篇关于如何将谓词表达式分解为查询?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!