.Net / Wcf bug: Cannot convert type 'System.DateTime[]' to 'System.DateTime'

@YaronNaveh

A common bug in .Net 2.0 may cause the following exception when compiling a proxy:

Unable to generate a temporary class (result=1).

error CS0030: Cannot convert type 'System.DateTime[]' to 'System.DateTime'

error CS0030: Cannot convert type 'System.DateTime[]' to 'System.DateTime'

error CS0029: Cannot implicitly convert type 'System.DateTime' to 'System.DateTime[]'

Microsoft did not fix this bug for Wcf so it also gives a similar exception:

There was an error in serializing body of message myRequest: 'Unable to generate a temporary class (result=1).
error CS0030: Cannot convert type 'System.DateTime[]' to 'System.DateTime'
error CS0030: Cannot convert type 'System.DateTime[]' to 'System.DateTime'
error CS0029: Cannot implicitly convert type 'System.DateTime' to 'System.DateTime[]'
error CS0029: Cannot implicitly convert type 'System.DateTime' to 'System.DateTime[]'
'.  Please see InnerException for more details.

Is this error limited to DateTime?

Not at all. You may also get any of these:

Cannot convert type 'System.String[]' to 'System.String'

Cannot convert type 'System.Int32[]' to 'System.Int32'

Cannot convert type 'System.Decimal[]' to 'System.Decimal'

Or with your custom types.

What does this error mean?

For you – nothing. This is a .Net/Wcf bug.

When does this happen?

When the following conditions apply:

1. The wsdl (schema) declares a complex type C.
2. C contains a single element with maxOccures>1 (an array).
3. The wsdl declares an element e of type X with maxOccures>1.

Example:

  <xsd:complexType name="C">
    <xsd:sequence>
      <xsd:element maxOccurs="unbounded" minOccurs="0" name="someDate" type="xsd:date"/>    
    </xsd:sequence>
  </xsd:complexType>

...

<xsd:element maxOccurs="unbounded" minOccurs="0" name="e" type="tns:C"/> 

What is the .Net bug?

The proxy will generate something like this:

        [XmlArrayAttribute]
        [XmlArrayItemAttribute("someDate", typeof(DateTime))]
        public DateTime[][] C {
        ...
        }

This line declares an array of type DateTime[][], but also specifies that each item of the array is of type DateTime. Which is clearly an error since the array items are of type DateTime[] (this is a multidimentional array).

This is the reason why the proxy does not compile.

Workaround #1

Manually edit the proxy and change “typeof(DateTime)” to “typeof(DateTime[])”. This workaround is also described here.

Limitation: This change will be overridden every time you regenerate the proxy (update web/service reference).

Workaround #2

We mentioned that the bug only happens when the type C has one child element. Why not add it some friend so it would not be alone? This means we need to manually edit the wsdl. For example:

<xsd:complexType name="C">
    <xsd:sequence>
      <xsd:element maxOccurs="unbounded" minOccurs="0" name="someDate" type="xsd:date"/>          
<xsd:element minOccurs="0" name="dummyFriend" type="xsd:string"/>     
    </xsd:sequence>
  </xsd:complexType>

Now you may say this changes the schema and makes our proxy not valid. But note we have added an optional field only (minOccurs=0) so as long as we do not assign any value to it in our client this change is non-breaking.

Now let’s see how the proxy generated from the updated wsdl looks like:

public partial class C {
        [XmlElementAttribute("dateValue", DataType="date")]
        public DateTime[] dateValue {
            ...
        }

    [XmlElementAttribute]
        public string dummyFriend {
            ...
        }
    }

Now .Net does not generate the multidimensional array in one shot, but declares the mediating type C.  The proxy will in turn use this type as an array:

public C[] e {…}

which gives us the multidimensional effect.

This compiles immediately so we’re free to go.

@YaronNaveh

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