Expression.Property的Expression.Convert类型

Question

我正在尝试转换一个参数表达式，但在转换为值类型时遇到了问题。下面是我的代码示例：

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;
}

我得到以下无效操作异常：

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

对此转换的任何帮助都将不胜感激。

Answer 1

我采用的解决方案是：

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;
}

请注意处理空值的Expression.Condition。

Answer 2

public class ExpressionUtils
{
    public static MethodCallExpression ConvertToType(
        ParameterExpression sourceParameter,
        PropertyInfo sourceProperty,
        TypeCode typeCode)
    {
        var sourceExpressionProperty = Expression.Property(sourceParameter, sourceProperty);
        var changeTypeMethod = typeof(Convert).GetMethod("ChangeType", new Type[] { typeof(object), typeof(TypeCode) });
        var callExpressionReturningObject = Expression.Call(changeTypeMethod, sourceExpressionProperty, Expression.Constant(typeCode));
        return callExpressionReturningObject;
    }
}

请注意，结果表达式是对Convert.ChangeType方法的调用，该方法将返回System.Object。

这是一个单元测试：

[TestClass]
public class UnitTest1
{
    private class MyClass
    {
        public string ValueAsString { get; set; }
    }

    [TestMethod]
    public void TestMethod1()
    {
        var parameter = Expression.Parameter(typeof(MyClass));
        var property = typeof(MyClass).GetProperty("ValueAsString");
        var lambdaBody = ExpressionUtils.ConvertToType(parameter, property, TypeCode.Decimal);
        var lambda = Expression.Lambda<Func<MyClass, object>>(lambdaBody, parameter);
        var valueAsDecimal = (decimal) lambda.Compile().Invoke(new MyClass { ValueAsString = "42" });
        Assert.AreEqual(42m, valueAsDecimal);
    }
}