如何将 LambdaExpression 转换为表达式<Func<T,bool> &gt; 在 C# 中

Question

我有以下代码，它根据我的SearchTerm输入在运行时生成LambdaExpression 。 我正在尝试构建一个动态的where子句。 但是我被困在如何从LambdaExpression转换为Expression<Func<T,bool>>

private static Expression<Func<T,bool>> GetSearchAppliedQuery(IEnumerable<SearchTerm> terms)
{
    var parameterExpression = ExpressionHelper.Parameter<T>();
    Expression finalExpression = Expression.Constant(true);
    Expression subExpression = Expression.Constant(false);

    // Build up the LINQ Expression backwards:
    // query = query.Where(x => x.Property == "Value" && (x.AnotherProperty == "Value" || x.SomeAnotherProperty == "Value"));

    foreach (var term in terms)
    {
        var hasMultipleTerms = term.EntityName?.Contains(',') ?? false;

        if (hasMultipleTerms)
        {
            var entityTerms = term.EntityName.Split(',');

            foreach (var entityTerm in entityTerms)
            {
                term.EntityName = entityTerm;

                // x => x.Property == "Value" || x.AnotherProperty == "Value"
                subExpression = Expression.OrElse(subExpression, GetComparisonExpression(term, parameterExpression));
            }
        }

        // x => x.Property == "Value" && x.AnotherProperty == "Value"
        finalExpression = Expression.AndAlso(finalExpression, hasMultipleTerms ? subExpression : GetComparisonExpression(term, parameterExpression));
    }

    // x => x.Property == "Value" && (x.AnotherProperty == "Value" || x.SomeAnotherProperty == "Value")
    var lambdaExpression = ExpressionHelper.GetLambda<T, bool>(parameterExpression, finalExpression);

    // How to do this conversion??
    Expression<Func<T,bool>> returnValue = ..??;

    return returnValue;
}

我正在尝试应用上述方法的结果来获取查询，如下所示：

public static IQueryable<T> GetQuery(IQueryable<T> inputQuery, ISpecification<T> specification)
{
    var query = inputQuery;

    // modify the IQueryable using the specification's criteria expression
    if (specification.Criteria != null)
    {
        query = query.Where(specification.Criteria);
    }

    ...
    return query;
}

所以我的最终查询看起来像，

query = query.Where(x => x.Property == "Value" && (x.AnotherProperty == "Value" || x.SomeAnotherProperty == "Value"))

编辑 1：根据@Ivan Stoev 的要求添加ExpressionHelper.GetLambda方法

public static class ExpressionHelper
{
    public static LambdaExpression GetLambda<TSource, TDest>(ParameterExpression obj, Expression arg)
    {
        return GetLambda(typeof(TSource), typeof(TDest), obj, arg);
    }

    public static LambdaExpression GetLambda(Type source, Type dest, ParameterExpression obj, Expression arg)
    {
        var lambdaBuilder = GetLambdaFuncBuilder(source, dest);
        return (LambdaExpression)lambdaBuilder.Invoke(null, new object[] { arg, new[] { obj } });
    }

    private static MethodInfo GetLambdaFuncBuilder(Type source, Type dest)
    {
        var predicateType = typeof(Func<,>).MakeGenericType(source, dest);
        return LambdaMethod.MakeGenericMethod(predicateType);
    }
}

我是否遗漏了一些非常基本的东西或做错了什么？ 请协助。

Answer 1

用于获取 lambda 表达式的ExpressionHelper.GetLambda<T, bool>方法隐藏了其实际类型，即所需的Expression<Func<T, bool>> ，因此您只需要使用强制转换运算符：

return (Expression<Func<T, bool>>)lambdaExpression;

或者更好的是，将ExpressionHelper.GetLambda<TSource, TDest>的结果类型更改为Expression<Func<TSource, TDest>> ，或者不使用该辅助方法 - 当您在编译时知道泛型类型参数时，只需使用一个如果通用Expression.Lambda方法（ ExpressionHelper.GetLambda<TSource, TDest>似乎相当于Expression.Lambda<Func<TSource, TDest>> ），例如

var lambdaExpression = Expression.Lambda<Func<T, bool>>(parameterExpression, finalExpression);