restsharp/RestSharp · error · FormatException

Couldn't parse the value of '{value}' into the '{prop.Name}'

Error message

Couldn't parse the value of '{value}' into the '{prop.Name}' property, because it isn't a type of '{prop.PropertyType}'.

What it means

Thrown during XML deserialization when a primitive property value from the XML cannot be converted (via ChangeType) to the property's declared type, producing a FormatException. The inner FormatException's original message is replaced with this contextual one naming the value and the target property/type.

Source

Thrown at src/RestSharp.Serializers.Xml/XmlDeserializer.cs:185

                    continue;
                }

                type = type.GetGenericArguments()[0].GetTypeInfo();
            }

            var asType = type.AsType();

            if (asType == typeof(bool)) {
                var toConvert = value.ToString()!.ToLower(Culture);

                prop.SetValue(x, XmlConvert.ToBoolean(toConvert), null);
            }
            else if (type.IsPrimitive) {
                try {
                    prop.SetValue(x, value.ChangeType(asType), null);
                }
                catch (FormatException ex) {
                    throw new FormatException(
                        $"Couldn't parse the value of '{value}' into the '{prop.Name}'" +
                        $" property, because it isn't a type of '{prop.PropertyType}'.",
                        ex.InnerException
                    );
                }
            }
            else if (type.IsEnum) {
                var converted = type.AsType().FindEnumValue(value.ToString(), Culture);

                prop.SetValue(x, converted, null);
            }
            else if (asType == typeof(Uri)) {
                var uri = new Uri(value.ToString(), UriKind.RelativeOrAbsolute);

                prop.SetValue(x, uri, null);
            }
            else if (asType == typeof(string)) {
                prop.SetValue(x, value, null);

View on GitHub (pinned to 6a50821692)

Solutions

1. Inspect the raw XML response content to see the offending value for the named property.
2. Change the property type to string or a nullable type so conversion does not fail, then parse manually.
3. Ensure the server returns values compatible with the property's declared primitive type.

Example fix

// before
public int Count { get; set; }

// after
public int? Count { get; set; }

try { var result = client.GetAsync<T>(req); } catch (DeserializationException ex) when (ex.InnerException is FormatException) { /* log raw XML, relax property types, or parse manually */ }

When it happens

Trigger: The XML element/attribute text cannot be parsed as the property's primitive type, e.g. an element mapped to an int contains 'abc', or a value mapped to a numeric type holds a non-numeric string, raising FormatException inside ChangeType.

Common situations: Schema drift where the server sends strings in numeric fields; locale/number-format mismatches; empty string in a non-nullable numeric field; boolean fields receiving non-boolean text other than those XmlConvert.ToBoolean handles.

Related errors

• Class cannot have two properties marked with SerializeAs(Con
• Response content is null
• The type must contain a public, parameterless constructor.
• If the type implements IEnumerable<T>, then it must contain
• Response content is null

AI-assisted analysis of restsharp/RestSharp@6a50821692 (2026-08-13). Data as JSON: /api/errors/1c02d2fea8f2a97f. Report an issue: GitHub.