如何创建一个表达式树来表示 C # 中的“ String.Contains(“ term”)”？

我刚刚开始使用表达式树，所以我希望这有意义。我试图创建一个表达式树来表示:

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

到目前为止，我得到了:

    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。

感谢你的帮助。

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小开
最佳答案

比如:

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);
}

你可能会发现 这个有帮助。

小开

这样吧:

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
小开

执行如下搜索:

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

跟踪字符串为:

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

我使用下面我创建的方法:

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);
}
小开

下面是如何创建字符串的表达式树。

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"));

https://raw.githubusercontent.com/xavierjohn/Its.Cqrs/e44797ef6f47424a1b145d69889bf940b5581eb8/Domain.Sql/CatchupEventFilter.cs的实际使用情况


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