如何创建表情 [英] How to create Expression

问题描述

我有3个变量:

String propertyName = "Title";
String propertyValue = "Bob";
Type propertyType = typeof(String);

如何构造表达式<Func<T, bool>>， 如果T对象具有属性标题?

How can I construct Expression <Func<T, bool>>, if T object has property Title?

我需要表达:

item => item.Title.Contains("Bob")

如果propertyType是bool，那么我需要

if propertyType is bool, then I need

item => item.OtherOproperty == false/true

以此类推...

推荐答案

此代码执行过滤并将结果存储在过滤后的数组中:

This code performs filtering and stores results in filtered array:

IQueryable<T> queryableData = (Items as IList<T>).AsQueryable<T>();

PropertyInfo propInfo = typeof(T).GetProperty("Title");
ParameterExpression pe = Expression.Parameter(typeof(T), "Title");
Expression left = Expression.Property(pe, propInfo);
Expression right = Expression.Constant("Bob", propInfo.PropertyType);
Expression predicateBody = Expression.Equal(left, right);

// Create an expression tree that represents the expression            
MethodCallExpression whereCallExpression = Expression.Call(
    typeof(Queryable),
    "Where",
    new Type[] { queryableData.ElementType },
    queryableData.Expression,
    Expression.Lambda<Func<T, bool>>(predicateBody, new ParameterExpression[] { pe }));

T[] filtered = queryableData.Provider.CreateQuery<T>(whereCallExpression).Cast<T>().ToArray();

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