调用Expression的Equal方法

Question

当我运行这段代码时

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

附加信息：类型的表达式'System.Int32'不能用于'System.Linq.Expressions.Expression'方法类型的参数'System.Linq.Expressions.BinaryExpression Equal(System.Linq.Expressions.Expression, System.Linq.Expressions.Expression)'

Answer 1

你不需要反思，除非我遗漏了一些东西：

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);

显然你可以手动调用该int.Equals(int)方法......

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，有一个微妙的区别：Expression.Equal将使用==运算符，而其他代码将使用Equals方法。

如果您确实想从字符串构建表达式：

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);

或者

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

使用Enum.Parse