我没有使用过那么多的表达式,所以我不能说我的意图是否有任何意义。我在网上找了一遍,但是一无所获。
假设我有一个这样的方法
public async Task<T> GetFirstWhereAsync(Expression<Func<T, bool>> expression)
{
// this would be a call to 3rd party dependency
return await SomeDataProvider
.Connection
.Table<T>()
.Where(expression)
.FirstOrDefaultAsync();
}
在my-other-code中,我可以调用它例如
private async Task<User> GetUser(Credentials credentials)
{
return await SomeDataSource
.GetFirstWhereAsync(u => u.UserName.Equals(credentials.UserName));
}
所以我将从匹配给定表达式的SomeDataProvider
中接收第一个User
。
我的实际问题是我如何修改GetFirstWhereAsync
,使它将一些SecretSauce
应用于传递给它的任何表达式?我可以在调用者(s)中这样做,但那将是丑陋的,没有多少乐趣。
如果我传入像
这样的表达式u => u.UserName.Equals(credentials.UserName);
p => p.productId == 1;
我想把这些修改成
u => u.UserName.Equals(SecretSauce.Apply(credentials.UserName));
p => p.productId == SecrectSauce.Apply(1);
您可以修改方法内部的表达式,但这有点复杂,您需要逐个处理。
下面的例子将处理从x => x.Id == 1
到x => x.Id == SecretSauce.Apply(1)
的修改
班
class User
{
public int Id { get; set; }
public string Name { get; set;}
public override string ToString()
{
return $"{Id}: {Name}";
}
}
<<h3>酱/h3>class SquareSauce
{
public static int Apply(int input)
{
// square the number
return input * input;
}
}
数据User[] user = new[]
{
new User{Id = 1, Name = "One"},
new User{Id = 4, Name = "Four"},
new User{Id = 9, Name = "Nine"}
};
方法User GetFirstWhere(Expression<Func<User, bool>> predicate)
{
//get expression body
var body = predicate.Body;
//get if body is logical binary (a == b)
if (body.NodeType == ExpressionType.Equal)
{
var b2 = ((BinaryExpression)body);
var rightOp = b2.Right;
// Sauce you want to apply
var methInfo = typeof(SquareSauce).GetMethod("Apply");
// Apply sauce to the right operand
var sauceExpr = Expression.Call(methInfo, rightOp);
// reconstruct equals expression with right operand replaced
// with "sauced" one
body = Expression.Equal(b2.Left, sauceExpr);
// reconstruct lambda expression with new body
predicate = Expression.Lambda<Func<User, bool>>(body, predicate.Parameters);
}
/*
deals with more expression type here using else if
*/
else
{
throw new ArgumentException("predicate invalid");
}
return user
.AsQueryable()
.Where(predicate)
.FirstOrDefault();
}
使用Console.WriteLine(GetFirstWhere(x => x.Id == 2).ToString());
该方法将把x => x.Id == 2
变为x => x.Id == SquareSauce.Apply(2)
,并将产生:
四4: