Expression.Property的Expression.Convert类型 [英] Expression.Convert type for Expression.Property

问题描述

我正在尝试转换参数表达式，但在转换为值类型时遇到了麻烦.以下是我的代码示例:

I'm trying to convert a Parameter expression and having trouble with converting to value types. Below is a sample of my code:

public static MemberExpression ConvertToType(ParameterExpression sourceParameter,
                                              PropertyInfo propertyInfo, 
                                              TypeCode typeCode)
{
    var sourceExpressionProperty = Expression.Property(sourceParameter, sourceProperty);

    //throws an exception if typeCode is a value type.
    Expression convertedSource = Expression.Convert(sourceExpressionProperty,
                                                Type.GetType("System." + typeCode));

    return convertedSource;
}

我收到以下无效的操作异常:

I get the following invalid operation exception:

No coercion operator is defined between types 'System.String' and 'System.Decimal'.

对于此转换的任何帮助，将不胜感激.

Any help with this conversion would be greatly appreciated.

推荐答案

我使用的解决方案是:

private static Expression GetConvertedSource(ParameterExpression sourceParameter,
                                             PropertyInfo sourceProperty, 
                                             TypeCode typeCode)
{
    var sourceExpressionProperty = Expression.Property(sourceParameter,
                                                       sourceProperty);

    var changeTypeCall = Expression.Call(typeof(Convert).GetMethod("ChangeType", 
                                                           new[] { typeof(object),
                                                            typeof(TypeCode) }),
                                                            sourceExpressionProperty,
                                                            Expression.Constant(typeCode)
                                                            );

    Expression convert = Expression.Convert(changeTypeCall, 
                                            Type.GetType("System." + typeCode));

    var convertExpr = Expression.Condition(Expression.Equal(sourceExpressionProperty,
                                            Expression.Constant(null, sourceProperty.PropertyType)),
                                            Expression.Default(Type.GetType("System." + typeCode)),
                                            convert);



    return convertExpr;
}

请注意Expression.Condition可以处理空值.

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