如何创建表达式树来表示C＃中的String.Contains（“term”）'？
问题描述：
我刚开始使用表达式树，所以我希望这是有道理的。我想创建一个表达式树表示：如何创建表达式树来表示C＃中的String.Contains（“term”）'？
t => t.SomeProperty.Contains("stringValue");
到目前为止我有：
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。
答
喜欢的东西：
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);
}
您可能会发现this帮助。
答
如何：
Expression<Func<string, string, bool>> expFunc = (name, value) => name.Contains(value);
在客户端代码：
bool result = expFunc.Compile()("FooBar", "Foo"); //result true
result = expFunc.Compile()("FooBar", "Boo"); //result false
答
执行类似于搜索：
ef.Entities.Where(entity => arr.Contains(entity.Name)).ToArray();
该跟踪字符串将是：
SELECT .... From Entities ... Where Name In ("abc", "def", "qaz")
我用我下面创建的方法：
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);
}
答
这里是criteria.Contains的样品（MyParam）
var method = typeof(Enumerable).GetRuntimeMethods().Single(m => m.Name == nameof(Enumerable.Contains) && m.GetParameters().Length == 2);
var containsMethod = method.MakeGenericMethod(typeof(string));
var doesContain = Expression.Call(containsMethod, Expression.Constant(criteria.ToArray()), Expression.Property(p, "MyParam"));
如果我想打不startWith“substring”？ – amd 2015-04-04 16:29:57