Create new listItem with multichoice field Sharepoint Rest Api V1

Question

I'm trying to create a new list item in a SharePoint document library, but I have an error when I set a value in a multi-choice field.

This is the body of the post request:

{
    "listItemCreateInfo": {
        "FolderPath": {
            "DecodedUrl": "......."
        },
        "UnderlyingObjectType": 1
    },
    "formValues": [
        {
            "FieldName": "MultiChoice",
            "FieldValue": [
                "Scelta n. 1",
                "Scelta n. 3"
            ],
        },       
        {
            "FieldName": "FileLeafRef",
            "FieldValue": "2021-000001010"
        },        
        {
            "FieldName": "ContentTypeId",
            "FieldValue": "0x0120D5200062FEFD4873814585B3EFD57010F1F8AE"
        }
    ],
    "bNewDocumentUpdate": false
}

And this is the error:

An unexpected 'StartObject' node was found when reading from the JSON reader. A 'PrimitiveValue' node was expected

I tried the following solutions:

    "FieldValue": [
        "Scelta n. 1",
        "Scelta n. 3"
    ],

    "FieldValue": "{'results':[
        'Scelta n. 1',
        'Scelta n. 3'
    ]}",

but they all failed.

How should I create the JSON?

Answer 1

Instead of setting FieldValue to an array, set it to a string with the choice values separated by ";#". In the example you give above the JSON would become:

{
    "listItemCreateInfo": {
        "FolderPath": {
            "DecodedUrl": "......."
        },
        "UnderlyingObjectType": 1
    },
    "formValues": [
        {
            "FieldName": "MultiChoice",
            "FieldValue": "Scelta n. 1;#Scelta n. 3"
        },       
        {
            "FieldName": "FileLeafRef",
            "FieldValue": "2021-000001010"
        },        
        {
            "FieldName": "ContentTypeId",
            "FieldValue": "0x0120D5200062FEFD4873814585B3EFD57010F1F8AE"
        }
    ],
    "bNewDocumentUpdate": false
}

This blog post shows the format that should be use for field values of different types: Update the value of the field using the validateUpdateListItem method.