Question

我正在为项目开发动态查询解决方案。我想避免一堆if / else或switch语句，只是为了更改这些查询的[DynamicFieldName]部分。

        IQueryable<MyDataType> allItems = (from item in Context.MyDataTypes select item);

        foreach (QueryEntry currentEntry in query.Fields)
        {
            allItems = allItems.Where(item => item.[DynamicFieldName] == currentEntry.Value);
        }

用户可以通过具有可变字段数的GUI建立查询。最后，他们还将有各种比较选择（小于，大于，等于，包含等），这些比较会因数据类型而异。

我可以使用哪种方法以可重用的方式以编程方式构建此程序？

Answer 1

看看这段代码：

public static class CustomQueryBuilder
{
    //todo: add more operations
    public enum Operator
    {
        Equal = 0,
        GreaterThan = 1,
        LesserThan = 2
    }

    public static IQueryable<T> Where<T>(this IQueryable<T> query, string property, Operator operation, object value)
    {
        //it's an item which property we are referring to
        ParameterExpression parameter = Expression.Parameter(typeof(T));
        //this stands for "item.property"
        Expression prop = Expression.Property(parameter, property);

        //wrapping our value to use it in lambda
        ConstantExpression constant = Expression.Constant(value);

        Expression expression;
        //creating the operation
        //todo: add more cases
        switch (operation)
        {
            case Operator.Equal:
                expression = Expression.Equal(prop, constant);
                break;
            case Operator.GreaterThan:
                expression = Expression.GreaterThan(prop, constant);
                break;
            case Operator.LesserThan:
                expression = Expression.LessThan(prop, constant);
                break;
            default:
                throw new ArgumentException("Invalid operation specified");
        }

        //create lambda ready to use in queries
        var lambda = Expression.Lambda<Func<T, bool>>(expression, parameter);

        return query.Where(lambda);
    }
}

用法

var users = context
    .Users
    .Where("Name", CustomQueryBuilder.Operator.Equal, "User")
    .ToList();

等于

var users = context
    .Users
    .Where(u => u.Name == "User")
    .ToList();

动态建立EF查询

1 个答案: