调用表达式的相等方法 [英] call Equal method of Expression

问题描述

当我运行此代码时

Expression left = Expression.Constant(10, typeof(int));
Expression right = Expression.Constant(10,typeof(int));

var method10 = typeof(Expression).GetMethod("Equal", new[] { typeof(Expression), typeof(Expression) });
Expression exp = Expression.Call(method10,left,right);
var lambda = Expression.Lambda<Func<bool>>(exp);
var compiled = lambda.Compile().DynamicInvoke();

我收到以下错误

其他信息:类型'System.Int32'的表达式不能用于方法'System.Linq.Expressions.BinaryExpression Equal(System.Linq.Expressions.Expression, System.Linq.Expressions.Expression)'

Additional information: Expression of type 'System.Int32' cannot be used for parameter of type 'System.Linq.Expressions.Expression' of method 'System.Linq.Expressions.BinaryExpression Equal(System.Linq.Expressions.Expression, System.Linq.Expressions.Expression)'

推荐答案

除非我缺少某些东西，否则您不需要反思:

You don't need reflection, unless I'm missing something:

Expression left = Expression.Constant(10, typeof(int));
Expression right = Expression.Constant(10,typeof(int));
Expression exp = Expression.Equal(left, right);
var lambda = Expression.Lambda<Func<bool>>(exp);

很显然，您可以手动调用int.Equals(int)方法...

Clearly you could manually call the int.Equals(int) method...

var method10 = left.Type.GetMethod("Equals", new[] { right.Type });
Expression exp = Expression.Call(left, method10, right);
var lambda = Expression.Lambda<Func<bool>>(exp);

，但请注意有一个细微的区别:Expression.Equal将使用==运算符，而其他代码将使用Equals方法.

but note that there is a subtle difference: Expression.Equal will use the == operator, while the other code will use the Equals method.

如果您真的想从字符串构建表达式:

If you really want to build an expression from a string:

string methodName = "Equal";
MethodInfo method10 = typeof(Expression).GetMethod(methodName, new[] { typeof(Expression), typeof(Expression) });
Expression exp = (Expression)method10.Invoke(null, new object[] { left, right });
var lambda = Expression.Lambda<Func<bool>>(exp);

或

ExpressionType type = (ExpressionType)Enum.Parse(typeof(ExpressionType), methodName);
var exp = Expression.MakeBinary(type, left, right);

使用Enum.Parse

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