如何创buildexpression式树来表示C＃中的String.Contains（“term”）？

我只是开始expression树，所以我希望这是有道理的。 我正在尝试创build一个expression式树来表示：

t => t.SomeProperty.Contains("stringValue"); 

到目前为止我已经得到：

  private static Expression.Lambda<Func<string, bool>> GetContainsExpression<T>(string propertyName, string propertyValue) { var parameterExp = Expression.Parameter(typeof(T), "type"); var propertyExp = Expression.Property(parameter, propertyName); var containsMethodExp = Expression.*SomeMemberReferenceFunction*("Contains", propertyExp) //this is where I got lost, obviously :) ... return Expression.Lambda<Func<string, bool>>(containsMethodExp, parameterExp); //then something like this } 

我只是不知道如何引用String.Contains（）方法。

帮助赞赏。

Answer 1

就像是：

 class Foo { public string Bar { get; set; } } static void Main() { var lambda = GetExpression<Foo>("Bar", "abc"); Foo foo = new Foo { Bar = "aabca" }; bool test = lambda.Compile()(foo); } static Expression<Func<T, bool>> GetExpression<T>(string propertyName, string propertyValue) { var parameterExp = Expression.Parameter(typeof(T), "type"); var propertyExp = Expression.Property(parameterExp, propertyName); MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) }); var someValue = Expression.Constant(propertyValue, typeof(string)); var containsMethodExp = Expression.Call(propertyExp, method, someValue); return Expression.Lambda<Func<T, bool>>(containsMethodExp, parameterExp); } 

你可能会觉得这很有帮助。

Answer 2

这个怎么样：

 Expression<Func<string, string, bool>> expFunc = (name, value) => name.Contains(value); 

在客户端代码中：

  bool result = expFunc.Compile()("FooBar", "Foo"); //result true result = expFunc.Compile()("FooBar", "Boo"); //result false 

Answer 3

要执行search，如：

 ef.Entities.Where(entity => arr.Contains(entity.Name)).ToArray(); 

跟踪string将是：

 SELECT .... From Entities ... Where Name In ("abc", "def", "qaz") 

我使用下面创build的方法：

 ef.Entities.Where(ContainsPredicate<Entity, string>(arr, "Name")).ToArray(); public Expression<Func<TEntity, bool>> ContainsPredicate<TEntity, T>(T[] arr, string fieldname) where TEntity : class { ParameterExpression entity = Expression.Parameter(typeof(TEntity), "entity"); MemberExpression member = Expression.Property(entity, fieldname); var containsMethods = typeof(Enumerable).GetMethods(BindingFlags.Static | BindingFlags.Public) .Where(m => m.Name == "Contains"); MethodInfo method = null; foreach (var m in containsMethods) { if (m.GetParameters().Count() == 2) { method = m; break; } } method = method.MakeGenericMethod(member.Type); var exprContains = Expression.Call(method, new Expression[] { Expression.Constant(arr), member }); return Expression.Lambda<Func<TEntity, bool>>(exprContains, entity); }