Expression.Property的Expression.Convert类型

Question

我正在尝试转换参数表达式，但无法转换为值类型。以下是我的代码示例：

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;
}

我收到以下无效的操作异常：

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

此转换的任何帮助将不胜感激。

Answer 1

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);
    }
}