抽象工厂模式默认类问题

你好我有一个类，它有一个带有一些条件检查的方法，并根据条件附加一个字符串。我试图用抽象工厂模式重构这段代码。

我的问题与默认表达式类有关。如果其他规则都不适用于相关的过滤器，那么我想应用默认的表达式规则。然而，在我的申请中；它取决于expressions列表中StandartExpression实例顺序的顺序。如果先添加，则不会检查其他规则。

我想我错过了一些关于这种模式的东西。你能帮我吗？

这是我的应用程序代码:

public class Filter
{
    private string _operator = "="; // =, !=, <,> ...
    public string Name { get; set; }
    public object Value { get; set; }
    public string Operator { get { return _operator; } set { _operator = value; } }
}
Filter filter = null;
StringBuilder whereClause = new StringBuilder();
for (int i = 0; i < array.Length; i++)
{
    filter = array[i];
    if (filter.Value.ToString().ToLower().Equals("null"))
    {
        whereClause.Append(filter.Name + " " + filter.Operator + " null ");
    }
    else if (filter.Operator.ToLower().Equals("like"))
    {
        whereClause.Append(filter.Name + ".Contains(@" + i + ")");
    }
    else
    {
        whereClause.Append(filter.Name + " " + filter.Operator + " @" + i);
    }
    whereClause.Append(" AND ");
}

现在，这是抽象的工厂模式代码：

public interface ICustomExpression
{
    bool ExpressionIsValid(Filter filter);
    string GetExpressionStr(Filter filter, int index);
}

public class StandardExpression : ICustomExpression
{
    public bool ExpressionIsValid(Filter filter)
    {
        return true;
    }
    public string GetExpressionStr(Filter filter, int index)
    {
        return filter.Name + " " + filter.Operator + " @" + index;
    }
}
public class LikeExpression : ICustomExpression
{
    public bool ExpressionIsValid(Filter filter)
    {
        return filter.Operator.ToLower().Equals("like");
    }
    public string GetExpressionStr(Filter filter, int index)
    {
        return filter.Name + ".Contains(@" + index + ")";
    }
}
public class NullExpression : ICustomExpression
{
    public bool ExpressionIsValid(Filter filter)
    {
        return filter.Value.ToString().ToLower().Equals("null");
    }
    public string GetExpressionStr(Filter filter, int index)
    {
        return filter.Name + " " + filter.Operator + " null ";
    }
}
public static class ExpressionFactory
{
    private static List<ICustomExpression> _expressions = null;
    public static List<ICustomExpression> Expressions
    {
        get
        {
            if (_expressions == null)
            {
                Build();
            }
            return _expressions;
        }
    }
    private static void Build()
    {
        _expressions = new List<ICustomExpression>();
        _expressions.Add(new NullExpression());
        _expressions.Add(new LikeExpression());
        _expressions.Add(new StandardExpression());
    }
}

这就是我在应用程序代码:中使用该结构的方式

StringBuilder whereClause = new StringBuilder();
Filter filter = null;
var array = filterList.ToArray();
for (int i = 0; i < array.Length; i++)
{
    filter = array[i];
    foreach (ICustomExpression exp in ExpressionFactory.Expressions)
    {
        if (exp.ExpressionIsValid(filter))
            whereClause.Append(exp.GetExpressionStr(filter, i));
    }

    whereClause.Append(" AND ");
}

public static class ExpressionFactory
{
    private static StandardExpression _standardExpression = new StandardExpression();
    private static ICustomExpression[] _specialExpressions = new []
    {
        (ICustomExpression)new NullExpression(),
        (ICustomExpression)new LikeExpression()
    };
    public static ICustomExpression GetBy(Filter filter)
    {
        var match = _specialExpressions.SingleOrDefault(e => e.ExpressionIsValid(filter));
        if (match == null)
            return _standardExpression;
        return match;
    }
}

for (int i = 0; i < array.Length; i++)
{
    filter = array[i];  
    var exp = ExpressionFactory.GetBy(filter);          
    whereClause.Append(exp.GetExpressionStr(filter, i));        
    whereClause.Append(" AND ");
}