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Im trying to apply a filter to a DataGrid in WPF and the filter property needs a Predicate

ex:

dataGrid1.Filter = p => p.A_Field_Table1.Contains(textBox.Text);

But my datagrid is being filled with reflection, so I only know the type of objects inside the datagrid at runtime.

Then I created a method that dynamically generates the Predicate< T > :

public static Predicate< T > GetPredicate< T >(string column, string valueP, T objSource, string table)
    {
        Type itemType = typeof(T);

        ParameterExpression predParam = Expression.Parameter(itemType, "p");
        Expression left = Expression.Property(predParam, itemType.GetProperty("A_" + column+ "_" + table));
        var valueStr= Expression.Constant(valueP);
        var typeOfStr = valueStr.Type;
        var containsMethod = typeOfStr.GetMethod("Contains", new [] { typeof(string) });
        var call = Expression.Call(left, containsMethod, valueStr);
        Func< T, bool > function = (Func< T, bool >)Expression.Lambda(call, new[] { predParam }).Compile();
        return new Predicate< T >(function);
    }

Then call this function on the interface:

var dataGridItem = dataGrid.Items[0];
dataGrid1.Filter = Class_X.GetPredicate(columnRefName,textBox.Text,dataGridItem,tableRefName);

But the generic method is throwing an exception saying that the type T is type of "object", even if objSource is type of Model.TableName.

I read some tutorials saying that T could not be resolved at runtime, then I should use "dynamic" instead of generic types.

I already tried using the "dynamic" type but I get a exception while casting the Lambda expression to Func< dynamic, bool>. Saying that I can't convert from < Model.TableName , bool > to < System.Object , bool >.

Is there an easier way to filter a datagrid that was filled by reflection?

4

1 回答 1

1

在这种情况下，您不能使用泛型。Filter是Func<object, bool>，所以：

public static Predicate<object> GetPredicate(string column, string valueP, object objSource, string table)
{
    Type itemType = objSource.GetType();

    ParameterExpression predParam = Expression.Parameter(typeof(object), "p");
    Expression left = Expression.Property(Expression.Convert(predParam, itemType), "A_" + column+ "_" + table);
    var valueStr= Expression.Constant(valueP);
    var typeOfStr = valueStr.Type;
    var containsMethod = typeOfStr.GetMethod("Contains", new [] { typeof(string) });

    var call = Expression.Call(left, containsMethod, valueStr);

    //To handle null values. It considers null == string.Empty
    //var left2 = Expression.Coalesce(left, Expression.Constant(string.Empty));
    //var call = Expression.Call(left2, containsMethod, valueStr);

    //If you want null values to be distinct from string.Empty, it's
    //much more complex. You'll need a temporary variable (left2)
    //where to put the value of the property, and then you can use the 
    //Expression.Condition (that is the ? : ternary operator) to 
    //test for null values
    //var left2 = Expression.Variable(typeof(string));
    //var assign = Expression.Assign(left2, left);
    //var notNull = Expression.NotEqual(left2, Expression.Constant(null));
    //var call = Expression.Call(left2, containsMethod, valueStr);
    //var condition = Expression.Condition(notNull, call, Expression.Constant(false));
    //var block = Expression.Block(new[] { left2 }, new Expression[] { assign, condition });

    Predicate<object> function = Expression.Lambda<Predicate<object>>(call, new[] { predParam }).Compile();
    return function;
}

“技巧”是在返回的函数中，参数被强制转换为“正确”类型（由 获得objSource.GetType()）

请注意，您没有测试null行属性中的值（NullReferenceException如果您尝试Contains在null属性上使用）

于 2013-08-27T14:36:24.893 回答