属性网格项并双击

我对布尔属性的自定义TypeConverter有问题。如果我有这门课:

public class MyClass {
    public string Name { get; set; }

    [System.ComponentModel.TypeConverter( typeof( BoolTypeConverter ) )]
    public bool Flag { get; set; }
}

我创建实例并在PropertyGrid中将其设置为SelectedObject—在用户双击属性网格项表单“Flag”属性之前,一切正常。双击后出现此消息:
_{(来源: tcks.wz.cz}

TypeConverter类看起来:

public class BoolTypeConverter : System.ComponentModel.TypeConverter {
    public const string TEXT_TRUE = "On";
    public const string TEXT_FALSE = "Off";
    public const string TEXT_NONE = "< none >";

    public override object CreateInstance( System.ComponentModel.ITypeDescriptorContext context, System.Collections.IDictionary propertyValues ) {
        object ret = base.CreateInstance( context, propertyValues );
        return ret;
    }
    public override bool GetCreateInstanceSupported( System.ComponentModel.ITypeDescriptorContext context ) {
        bool ret = base.GetCreateInstanceSupported( context );
        return ret;
    }
    public override bool IsValid( System.ComponentModel.ITypeDescriptorContext context, object value ) {
        bool ret;
        if ( value is string ) {
            string tmpValue = value.ToString().Trim();

            if ( string.Compare( tmpValue, TEXT_NONE, StringComparison.InvariantCultureIgnoreCase ) == 0 ) {
                ret = true;
            }
            else if ( string.Compare( tmpValue, TEXT_TRUE, StringComparison.InvariantCultureIgnoreCase ) == 0 ) {
                ret = true;
            }
            else if ( string.Compare( tmpValue, TEXT_FALSE, StringComparison.InvariantCultureIgnoreCase ) == 0 ) {
                ret = true;
            }
            else {
                bool blValue;
                ret = bool.TryParse( tmpValue, out blValue );
            }
        }
        else {
            ret = base.IsValid( context, value );
        }

        return ret;
    }

    public override bool CanConvertFrom( System.ComponentModel.ITypeDescriptorContext context, Type sourceType ) {
        bool ret = false;
        if ( sourceType == typeof( string ) ) {
            ret = true;
        }
        else {
            ret = base.CanConvertFrom( context, sourceType );
        }

        return ret;
    }
    public override object ConvertFrom( System.ComponentModel.ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value ) {
        object ret = null;

        bool converted = false;
        if ( value is string ) {
            string tmpValue = value.ToString().Trim();
            if ( string.Compare( tmpValue, TEXT_NONE, StringComparison.InvariantCultureIgnoreCase ) == 0
                || string.IsNullOrEmpty( tmpValue ) ) {
                ret = null;
                converted = true;
            }
            else if ( string.Compare( tmpValue, TEXT_TRUE, StringComparison.InvariantCultureIgnoreCase ) == 0 ) {
                ret = true;
                converted = true;
            }
            else if ( string.Compare( tmpValue, TEXT_FALSE, StringComparison.InvariantCultureIgnoreCase ) == 0 ) {
                ret = false;
                converted = true;
            }
            else {
                bool blValue;
                if ( converted = bool.TryParse( tmpValue, out blValue ) ) {
                    ret = blValue;
                }
            }
        }

        if ( false == converted ) {
            ret = base.ConvertFrom( context, culture, value );
        }
        return ret;
    }

    public override bool CanConvertTo( System.ComponentModel.ITypeDescriptorContext context, Type destinationType ) {
        bool ret = false;
        if ( destinationType == typeof( bool ) ) {
            ret = true;
        }
        else {
            ret = base.CanConvertTo( context, destinationType );
        }

        return ret;
    }
    public override object ConvertTo( System.ComponentModel.ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, Type destinationType ) {
        object ret = null;

        bool converted = false;
        if ( destinationType == typeof( string ) ) {
            if ( null == value ) {
                ret = TEXT_NONE;
                converted = true;
            }
            else if ( value is bool? || value is bool ) {
                if ( (bool)value ) { ret = TEXT_TRUE; }
                else { ret = TEXT_FALSE; }

                converted = true;
            }
            else if ( value is string ) {
                ret = value;
                converted = true;
            }
        }
        if ( false == converted ) {
            ret = base.ConvertTo( context, culture, value, destinationType );
        }
        return ret;
    }

    public override StandardValuesCollection GetStandardValues( System.ComponentModel.ITypeDescriptorContext context ) {
        StandardValuesCollection ret;
        Type tpProperty = context.PropertyDescriptor.PropertyType;
        if ( tpProperty == typeof( bool ) ) {
            ret = new StandardValuesCollection( new string[]{
            TEXT_TRUE, TEXT_FALSE
        } );
        }
        else if ( tpProperty == typeof( bool? ) ) {
            ret = new StandardValuesCollection( new string[]{
                TEXT_TRUE, TEXT_FALSE, TEXT_NONE
            } );
        }
        else {
            ret = new StandardValuesCollection( new string[0] );
        }

        return ret;
    }
    public override bool GetStandardValuesSupported( System.ComponentModel.ITypeDescriptorContext context ) {
        bool ret;
        Type tpProperty = context.PropertyDescriptor.PropertyType;
        if ( tpProperty == typeof( bool ) || tpProperty == typeof( bool? ) ) {
            ret = true;
        }
        else {
            ret = false;
        }

        return ret;
    }
}

这种行为让用户非常困惑。我怎样才能预防呢?

谢谢