Question

我是Lambda Expressions的新手，我在获取我想要的输出时遇到了一些问题。更具体地说，我无法让LINQ包含部分Lambda表达式。

这是我到目前为止所拥有的：

// The following line is what I want for my Final Result
var distinctValues = db.Equipment.Where(m => m.Model != null && m.Model.Contains(SearchTerm)).Select(m => m.Model).Distinct().ToList();

var queryableData = db.Equipment.AsQueryable();

var parameterExpression = Expression.Parameter(typeof(Equipment), Column);
var constant = Expression.Constant(null);

var property = Expression.Property(parameterExpression, Column);
var expression1 = Expression.NotEqual(property, constant);
var expression2 = Expression.Equal(property,constant);

Expression callContains = Expression.Call(typeof(Queryable), "Contains", new[] { <string> } );

var lambda1 = Expression.Lambda<Func<Equipment, bool>>(expression1, parameterExpression);
var lambda2 = Expression.Lambda<Func<Equipment, string>>(property, parameterExpression);
var lambda3 = Expression.Lambda<Func<Equipment, bool>>(callContains, parameterExpression);
var compiledLambda1 = lambda1.Compile();
var compiledLambda2 = lambda2.Compile();

我正在传入任何属性和搜索词，我想生成上面的LINQ查询的等价物作为以“var distinctValues”开头的Lambda表达式。我遇到以“Expression callContains”开头的行有问题。

非常感谢任何帮助。

谢谢，

皮特

Answer 1

Contains是string.Contains。所以，给定：

static readonly MethodInfo contains = (from x in typeof(string).GetMethods(BindingFlags.Instance | BindingFlags.Public)
                                       where x.Name == nameof(string.Contains) && !x.IsGenericMethod
                                       let pars = x.GetParameters()
                                       where pars.Length == 1 &&
                                           pars[0].ParameterType == typeof(string)
                                       select x).Single();

然后.Where()表达式树是：

// m => 
var parameterExpression = Expression.Parameter(typeof(Equipment), "m");

// m.Model
var modelProperty = Expression.Property(parameterExpression, nameof(Equipment.Model));
// m.Model != null
var nonNullExpression = Expression.NotEqual(modelProperty, Expression.Constant(null, typeof(string)));

// SearchTerm
var searchTermConstant = Expression.Constant(SearchTerm);

// m.Model.Contains(SearchTerm)
var containsExpression = Expression.Call(modelProperty, contains, searchTermConstant);

// m.Model != null && m.Model.Contains(SearchTerm)
var andExpression = Expression.AndAlso(nonNullExpression, containsExpression);

// m => ...
var lambda = Expression.Lambda<Func<Equipment, bool>>(andExpression, parameterExpression);

Answer 2

谢谢Xanatos。在你的帮助下，我能够解决我的问题。这是我提出的最终解决方案。只是一些小的改变：

  // m => 
                var parameterExpression = Expression.Parameter(typeof(Equipment), "m");

                // m.Model
                var modelProperty = Expression.Property(parameterExpression, Column);
                // m.Model != null
                var nonNullExpression = Expression.NotEqual(modelProperty, Expression.Constant(null, typeof(string)));

                // SearchTerm
                var searchTermConstant = Expression.Constant(SearchTerm);

                // m.Model.Contains(SearchTerm)
                var containsExpression = Expression.Call(modelProperty, "contains", null, searchTermConstant);

                // m.Model != null && m.Model.Contains(SearchTerm)
                var andExpression = Expression.AndAlso(nonNullExpression, containsExpression);

                var lambda1 = Expression.Lambda<Func<Equipment, string>>(modelProperty, parameterExpression);
                var lambda2 = Expression.Lambda<Func<Equipment, bool>>(andExpression, parameterExpression);

                var compiledLambda1 = lambda1.Compile();
                var compiledLambda2 = lambda2.Compile();

                var distinctValues1 = db.Equipment.Where(compiledLambda2).Select(compiledLambda1).Distinct().ToList();

首先，我对containsExpression使用了不同的重载。其次，我想将model属性作为参数。基本上，我想传入一个变量属性（Column）和变量searchterm。就是这样。

包含不起作用的Lambda表达式

2 个答案: