Expression.Convert type for Expression.Property

问题:

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;
}

I get the following invalid operation exception:

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

Any help with this conversion would be greatly appreciated.

回答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;
    }
}

Note that the resulting expression is a call to Convert.ChangeType method, which will return System.Object.

Here is a unit test:

[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);
    }
}

回答2:

The solution I went with was:

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;
}

Note the Expression.Condition to handle nulls.