动态构建 EF 查询

Question

I am working on a dynamic query solution for a project. I want to avoid a bunch of if/else or switch statements just to change the [DynamicFieldName] part of these queries.

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

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

The user gets to build the query via the GUI that has a variable number of fields. In the end, they will also have a variety of comparisons to choose from (Less than, greater than, equal, contains, etc.) that vary by data type.

What method can I use to build this programatically in a nice reusable fashion?

Answer 1

Have a look at this code:

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);
    }
}

Usage

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

Which is equal to

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