[英]Create an expression tree in c#
I am attempting to create a dynamic query using expression trees in LINQ to represent the following query 我试图在LINQ中使用表达式树创建一个动态查询来表示以下查询
WageConstIns.Where(WageConstIn => WageConstIn.Serialno.ToString().StartsWith("2800"));
I have attempted to create it like so: 我试图像这样创建它:
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"));
I am getting an error during runtime. 我在运行时遇到错误。 How can the above query best be built using expression trees? 如何使用表达式树最好地构建上述查询?
The easiest way would be to just declare it as an Expression<Func<...>>
最简单的方法是将其声明为Expression<Func<...>>
public static class Program {
public static void Main() {
Expression<Func<DummyClass, Boolean>> predicate = WageConstIn => WageConstIn.Serialno.ToString().StartsWith("2800");
}
}
But if you want to construct it using different Expressions... 但是如果你想用不同的表达式构建它...
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 }
)
);
}
}
Most people [that I've talked to] who enter the domain of expression trees are usually satisfied with the System.Linq.Dynamic functionality. 进入表达式树域的大多数[我已经与之交谈]的人通常对System.Linq.Dynamic功能感到满意。 (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. (这可以被滥用到很多不同的方式。)这个纯粹的awesomeness代码片段是Visual Studio示例代码的一部分,可能已经隐藏在你的硬盘驱动器的某个地方。
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