C#创建一个表达式树 [英] Create an expression tree in c#
问题描述
我试图用表达式树在LINQ代表下面的查询来创建一个动态查询
I am attempting to create a dynamic query using expression trees in LINQ to represent the following query
WageConstIns.Where(WageConstIn => WageConstIn.Serialno.ToString().StartsWith("2800"));
我试图像这样创造的:
I have attempted to create it like so:
MemberExpression le1 = LinqExpression.Property(paramExp, "Serialno");
MethodCallExpression le2 = LinqExpression.Call(le1, typeof(string).GetMethod("ToString", System.Type.EmptyTypes));
ConstantExpression le3 = LinqExpression.Constant("2800");
MethodCallExpression le4 = LinqExpression.Call(le2, typeof(string).GetMethod("StartsWith"));
我在运行时得到一个错误。如何能在上面的查询最好使用表达式目录树建?
I am getting an error during runtime. How can the above query best be built using expression trees?
推荐答案
最简单的方法是只把它声明为一个表达式来; Func键< ...>>
The easiest way would be to just declare it as an Expression<Func<...>>
public static class Program {
public static void Main() {
Expression<Func<DummyClass, Boolean>> predicate = WageConstIn => WageConstIn.Serialno.ToString().StartsWith("2800");
}
}
但是,如果你想用不同的表现方式来构建它。 ..
But if you want to construct it using different Expressions...
public static class Program {
public static void Main() {
var param = Expression.Parameter(typeof(DummyClass), "WageConstIn");
var constValue = Expression.Constant("2800");
// WageConstIn => WageConstIn.Serialno.ToString().StartsWith(...)
var first = Expression.Lambda(
parameters: param,
body: Expression.Call(
instance: Expression.Call(
instance: Expression.Property(param, "Serialno"),
methodName: "ToString",
typeArguments: null,
arguments: null
),
methodName: "StartsWith",
typeArguments: null,
arguments: new[] { constValue }
)
);
// WageConstIn => Convert.ToString(WageConstIn.Serialno).StartsWith(...)
var second = Expression.Lambda(
parameters: param,
body: Expression.Call(
instance: Expression.Call(
type: typeof(Convert),
methodName: "ToString",
typeArguments: null,
arguments: new[] { Expression.Property(param, "Serialno") }
),
methodName: "StartsWith",
typeArguments: null,
arguments: new[] { constValue }
)
);
}
}
大多数人[我已经说过了]谁输入表达式树的域名通常是满意的 System.Linq.Dynamic 功能。 (这可能会被滥用到了很多不同的方式。)这纯迷死代码片段是Visual Studio示例代码的一部分,很可能藏在某个地方在你的硬盘了。
Most people [that I've talked to] who enter the domain of expression trees are usually satisfied with the System.Linq.Dynamic functionality. (Which can be abused into a lot of different ways.) This code snippet of pure awesomeness is a part of the Visual Studio sample code, probably hiding somewhere on your harddrive already.
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